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颜色的使用是人类表达的基本,孩子们开始画画甚至都是从实验颜色开始的。或许这也是当人们学习网页开发时,第一件事就是尝试各种颜色的原因。使用 CSS 有非常多的方法给你的 HTML元素 设置的颜色以创建你期望的外观。这篇文章则是一篇入门教程,来介绍将CSS颜色应用到HTML的各种不同的方法。

幸运的是,为HTML添加颜色是非常简单的,你几乎可以为任何东西添加颜色!

我们将会介绍在使用颜色时需要知道的:

然后,我们将会简短的讨论 如何明智的使用颜色:如何选择期望的颜色,如何兼顾不同视觉能力的人的需要。

 

可拥有颜色的事物

站在元素的层面来看,HTML里面的任何东西都可以拥有颜色。我们可以聚焦一下那些被着色在元素里的东西,比如文本或边框等等。对于每一个着色,我们都能看到一个控制他们颜色的CSS的属性列表。

站在基本的层面来看,color 属性定义了HTML元素内容的前景色,而 background-color 则定义了背景色。二者几乎可以被应用在任何元素汇总。

文本

每当元素被呈现时,下面的这些属性将决定这文本的颜色、背景色、文本装饰的颜色。

color
这个颜色将被应用于文字本身和任何文本装饰(如下划线、上划线、删除线等等)。
background-color
文本的背景色。
text-shadow
配置一个文本阴影。其中有一个选项是配置阴影的基本颜色(与背景色模糊和混合基于其他属性)。详情参见:Text drop shadows in [Page not yet written]
text-decoration-color
默认情况下,文本装饰(如下划线、删除线)是使用 color 属性里面的颜色。不过,你也可以使用 text-decoration-color 属性来指定一个不同的颜色来覆盖这个行为。
text-emphasis-color
当着色文本中每个字符附近的强调符号时的颜色,它主要被用在某些东亚语言中。
caret-color
当绘制 caret (有时被称作文本输入光标)时使用的颜色。这个将仅在有编辑功能的元素中起作用,如 <input><textarea>、拥有 contenteditable 属性的元素。

盒模型

任何元素都是一个装有内容的盒子,而不论盒子里装了什么,都会有背景和边框。

Borders
你可以通过 Borders 章节来看到一个可以设置盒子边框颜色的CSS属性列表。
background-color
背景色将会被显示在元素中没有前景内容的区域内。
column-rule-color
这个颜色将会被应用在文本被分栏时的分割线上。
outline-color
这个颜色将会被应用在在元素周围的 outline 中。outline 和 border有一些不同,outline 不需要在文档中占用空间(所以它可能会和其他内容重叠)。 outline 通常被使用在焦点指示,展示元素收到了输入事件。

边框

任何元素的周围都可以拥有一个边框环绕。一个基本的边框是一条线围绕着元素内容的边缘,有关于元素及其边框的更多信息参见 Box properties in 框模型,关于如何给边框应用样式可以参考 Styling borders using CSS

你可以使用快捷属性 border 来一次性配置有关于边框所有属性(不仅仅包含颜色,还有 widthstyle(实线、虚线……) 等等)

border-color
指定一个单一的颜色应用到元素的所有边框。
border-left-color, border-right-color, border-top-color, and border-bottom-color
设置对应方向的元素边框的颜色。
border-block-start-color and border-block-end-color
With these, you can set the color used to draw the borders which are closest to the start and end of the block the border surrounds. In a left-to-right writing mode (such as the way English is written), the block start border is the top edge and the block end is the bottom. This differs from the inline start and end, which are the left and right edges (corresponding to where each line of text in the box begins and ends).
border-inline-start-color and border-inline-end-color
These let you color the edges of the border closest to to the beginning and the end of the start of lines of text within the box. Which side this is will vary depending on the writing-mode, direction, and text-orientation properties, which are typically (but not always) used to adjust text directionality based on the language being displayed. For example, if the box's text is being rendered right-to-left, then the border-inline-start-color is applied to the right side of the border.

其他使用颜色的情景

CSS 不是唯一一个支持颜色的Web技术,这里有一些在Web中可用并支持颜色的其他图形技术。

Canvas API
可以让你在 <canvas> 中绘制2D的点阵图。详细可参考我们的 Canvas 教程
SVG (可伸缩的矢量图)
可以让你使用命令来画形状、图案、线条来生产出一个图片。SVG命令是由XML写成的,并且SVG可以直接嵌入在网页中,或像其他图片一样通过 <img> 引用。
WebGL
WebGL(Web Graphics Library)是一个可以在Web中绘制高性能2D和3D图形的 OpenGL ES-based API 。可以通过 Learn WebGL for 2D and 3D 来找到更多信息。

How to describe a color

In order to represent a color in CSS, you have to find a way to translate the analog concept of "color" into a digital form that a computer can use. This is typically done by breaking down the color into components, such as how much of each of a set of primary colors to mix together, or how bright to make the color. As such, there are several ways you can describe color in CSS.

For more detailed discussion of each of the color value types, see the reference for the CSS <color> unit.

Keywords

A set of standard color names have been defined, letting you use these keywords instead of numeric representations of colors if you choose to do so and there's a keyword representing the exact color you want to use. Color keywords include the standard primary and secondary colors (such as red, blue, or orange), shades of gray (from black to white, including colors like darkgray and lightgrey), and a variety of other blended colors including lightseagreen, cornflowerblue, and rebeccapurple.

See Color keywords in <color> for a list of all available color keywords.

RGB values

There are three ways to represent an RGB color in CSS.

Hexadecimal string notation

Hexadecimal string notation represents a color using hexadecimal digits to represent each of the color components (red, green, and blue). It may also include a fourth component: the alpha channel (or opacity). Each color component can be represented as a number between 0 and 255 (0x00 and 0xFF) or, optionally, as a number between 0 and 15 (0x0 and 0xF). All components must be specified using the same number of digits. If you use the single-digit notation, the final color is computed by using each component's digit twice; that is, "#D" becomes "#DD" when drawing.

A color in hexadecimal string notation always begins with the character "#". After that come the hexadecimal digits of the color code. The string is case-insensitive.

"#rrggbb"
Specifies a fully-opaque color whose red component is the hexadecimal number 0xrr, green component is 0xgg, and blue component is 0xbb.
"#rrggbbaa"
Specifies a color whose red component is the hexadecimal number 0xrr, green component is 0xgg, and blue component is 0xbb. The alpha channel is specified by 0xaa; the lower this value is, the more translucent the color becomes.
"#rgb"
Specifies a color whose red component is the hexadecimal number 0xrr, green component is 0xgg, and blue component is 0xbb.
"#rgba"
Specifies a color whose red component is the hexadecimal number 0xrr, green component is 0xgg, and blue component is 0xbb. The alpha channel is specified by 0xaa; the lower this value is, the more translucent the color becomes.

As an example, you can represent the opaque color bright blue as "#0000ff" or "#00f". To make it 25% opaque, you can use "#0000ff44" or "#00f4".

RGB functional notation

RGB (Red/Green/Blue) functional notation, like hexadecimal string notation, represents colors using their red, green, and blue components (as well as, optionally, an alpha channel component for opacity). However, instead of using a string, the color is defined using the CSS function rgb(). This function accepts as its input parameters the values of the red, green, and blue components and an optional fourth parameter, the value for the alpha channel.

Legal values for each of these parameters are:

red, green, and blue
Each must be an <integer> value between 0 and 255 (inclusive), or a <percentage> from 0% to 100%.
alpha
The alpha channel is a number between 0.0 (fully transparent) and 1.0 (fully opaque). You can also specify a percentage where 0% is the same as 0.0 and 100% is the same as 1.0.

For example, a bright red that's 50% opaque can be represented as rgb(255, 0, 0, 0.5) or rgb(100%, 0, 0, 50%).

HSL functional notation

Designers and artists often prefer to work using the HSL (Hue/Saturation/Luminosity) color method. On the web, HSL colors are represented using HSL functional notation. The hsl() CSS function is very similar to the rgb() function in usage otherwise.

HSL color cylinder
Figure 1. An HSL color cylinder. Hue defines the actual color based on the position along a circular color wheel representing the colors of the visible spectrum. Saturation is a percentage of how much of the way between being a shade of gray and having the maximum possible amount of the given hue. As the value of luminance (or lightness) increases, the color transitions from the darkest possible to the brightest possible (from black to white). Image courtesy of user SharkD on Wikipedia, distributed under the CC BY-SA 3.0 license.

The value of the hue (H) component of an HSL color is an angle from red around through yellow, green, cyan, blue, and magenta (ending up back at red again at 360°) that identifies what the base color is. The value can be specified in any <angle> unit supported by CSS, including degrees (deg), radians (rad), gradians (grad), or turns (turn). But this doesn't control how vivid or dull, or how bright or dark the color is.

The saturation (S) component of the color specifies what percentage of the final color is comprised of the specified hue. The rest is defined by the grey level provided by the luminance (L) component.

Think of it like creating the perfect paint color:

  1. You start with base paint that's the maximum intensity possible for a given color, such as  the most intense blue that can be represented by the user's screen. This is the hue (H) component: a value representing the angle around the color wheel for the vivid hue we want to use as our base.
  2. Then select a greyscale paint that corresponds how bright you want the color to be; this is the luminance. Do you want it to be very bright and nearly white, or very dark and closer to black, or somewhere in between? This is specified using a percentage, where 0% is perfectly black and 100% is perfectly white. (regardless of the saturation or hue). In between values are a literal grey area.
  3. Now that you have a grey paint and a perfectly vivid color, you need to mix them together. The saturation (S) component of the color indicates what percentage of the final color should be comprised of that perfectly vivid color. The rest of the final color is made up of the grey paint that represents the saturation.

You can also optionally include an alpha channel, to make the color less than 100% opaque.

Here are some sample colors in HSL notation:

Note that when you omit the hue's unit, it's assumed to be in degrees (deg).

Using color

Now that you know what CSS properties exist that let you apply color to elements and the formats you can use to describe colors, you can put this together to begin to make use of color. As you may have seen from the list under Things that can have color, there are plenty of things you can color with CSS. Let's look at this from two sides: using color within a stylesheet, and adding and changing color using JavaScript code to alter the styles of elements.

Specifying colors in stylesheets

The easiest way to apply color to elements—and the way you'll usually do it—is to simply specify colors in the CSS that's used when rendering elements. While we won't use every single property mentioned previously, we'll look at a couple of examples. The concept is the same anywhere you use color.

Let's take a look at an example, starting by looking at the results we're trying to achieve:

HTML

The HTML responsible for creating the above example is shown here:

<div class="wrapper">
  <div class="box boxLeft">
    <p>
      This is the first box.
    </p>
  </div>
  <div class="box boxRight">
    <p>
      This is the second box.
    </p>
  </div>
</div>

This is pretty simple, using a <div> as a wrapper around the contents, which consists of two more <div>s, each styled differently with a single paragraph (<p>) in each box.

The magic happens, as usual, in the CSS, where we'll apply colors define the layout for the HTML above.

CSS

We'll look at the CSS to create the above results a piece at a time, so we can review the interesting parts one by one.

.wrapper {
  width: 620px;
  height: 110px;
  margin: 0;
  padding: 10px;
  border: 6px solid mediumturquoise;
}

The .wrapper class is used to assign styles to the <div> that encloses all of our other content. This establishes thesize of the container using width and height as well as its margin and padding.

Of more interest to our discussion here is the use of the border property to establish a border around the outside edge of the element. This border is a solid line, 6 pixels wide, in the color mediumturquoise.

Our two colored boxes share a number of properties in common, so next we establish a class, .box, that defines those shared properties:

.box {
  width: 290px;
  height: 100px;
  margin: 0;
  padding: 4px 6px;
  font: 28px "Marker Felt", "Zapfino", cursive;
  display: flex;
  justify-content: center;
  align-items: center;
}

In brief, .box establishes the size of each box, as well as the configuration of the font used within. We also take advantage of CSS Flexbox to easily center the contents of each box. We enable flex mode using display: flex, and set both justify-content and align-items to center. Then we can create a class for each of the two boxes that defines the propeties that differ between the two.

.boxLeft {
  float: left;
  background-color: rgb(245, 130, 130);
  outline: 2px solid darkred;
}

The .boxLeft class—which, cleverly, is used to style the box on the left—floats the box to the left, then sets up the colors:

  • The box's background color is set by changing the value of the CSS background-color property to rgb(245, 130, 130).
  • An outline is defined for the box. Unlike the more commonly used border, outline doesn't affect layout at all; it draws over the top of whatever may happen to be outside the element's box instead of making room as border does. This outline is a solid, dark red line that's two pixels thick. Note the use of the darkred keyword when specifying the color.
  • Notice that we're not explicitly setting the text color. That means the value of color will be inherited from the nearest containing element that defines it. By default, that's black.
.boxRight {
  float: right;
  background-color: hsl(270deg, 50%, 75%);
  outline: 4px dashed rgb(110, 20, 120);
  color: hsl(0deg, 100%, 100%);
  text-decoration: underline wavy #88ff88;
  text-shadow: 2px 2px 3px black;
}

Finally, the .boxRight class describes the unique properties of the box that's drawn on the right. It's configured to float the box to the right so that it appears next to the previous box. Then the following colors are established:

  • The background-color is set using the HSL value specified using hsl(270deg, 50%, 75%). This is a medium purple color.
  • The box's outline is used to specify that the box should be enclosed in a four pixel thick dashed line whose color is a somewhat deeper purple (rgb(110, 20, 120)).
  • The foreground (text) color is specified by setting the color property to hsl(0deg, 100%, 100%). This is one of many ways to specify the color white.
  • We add a green wavy line under the text with text-decoration.
  • Finally, a bit of a shadow is added to the text using text-shadow. Its color parameter is set to black.

Letting the user pick a color

There are many situations in which your web site may need to let the user select a color. Perhaps you have a customizable user interface, or you're implementing a drawing app. Maybe you have editable text and need to let the user choose the text color. Or perhaps your app lets the user assign colors to folders or items. Although historically it's been necessary to implement your own color picker, HTML now provides support for browsers to provide one for your use through the <input> element, by using "color" as the value of its type attribute.

The <input> element represents a color only in the hexadecimal string notation covered above.

Example: Picking a color

Let's look at a simple example, in which the user can choose a color. As the user adjusts the color, the border around the example changes to reflect the new color. After finishing up and picking the final color, the color picker's value is displayed.

On macOS, you indicate that you've finalized selection of the color by closing the color picker window.

HTML

The HTML here creates a box that contains a color picker control (with a label created using the <label> element) and an empty paragraph element (<p>) into which we'll output some text from our JavaScript code.

<div id="box">
  <label for="colorPicker">Border color:</label>
  <input type="color" value="#8888ff" id="colorPicker">
  <p id="output"></p>
</div>

CSS

The CSS simply establishes a size for the box and some basic styling for appearances. The border is also established with a 2-pixel width and a border color that won't last, courtesy of the JavaScript below...

#box {
  width: 500px;
  height: 200px;
  border: 2px solid rgb(245, 220, 225);
  padding: 4px 6px;
  font: 16px "Lucida Grande", "Helvetica", "Arial", "sans-serif"
}

JavaScript

The script here handles the task of updating the starting color of the border to match the color picker's value. Then two event handlers are added to deal with input from the <input type="color"> element.

let colorPicker = document.getElementById("colorPicker");
let box = document.getElementById("box");
let output = document.getElementById("output");

box.style.borderColor = colorPicker.value;

colorPicker.addEventListener("input", function(event) {
  box.style.borderColor = event.target.value;
}, false);

colorPicker.addEventListener("change", function(event) {
  output.innerText = "Color set to " + colorPicker.value + ".";
}, false);

The input event is sent every time the value of the element changes; that is, every time the user adjusts the color in the color picker. Each time this event arrives, we set the box's border color to match the color picker's current value.

The change event is received when the color picker's value is finalized. We respond by setting the contents of the <p> element with the ID "output" to a string describing the finally selected color.

Using color wisely

Making the right choices when selecting colors when designing a web site can be a tricky process, especially if you aren't well-grounded in art, design, or at least basic color theory. The wrong color choice can render your site unattractive, or even worse, leave the content unreadable due to problems with contrast or conflicting colors. Worse still, if using the wrong colors can result in your content being outright unusable by people withcertain vision problems, particularly color blindness.

Finding the right colors

Coming up with just the right colors can be tricky, especially without training in art or design. Fortunately, there are tools available that can help you. While they can't replace having a good designer helping you make these decisions, they can definitely get you started.

Base color

The first step is to choose your base color. This is the color that in some way defines your web site or the subject matter of the site. Just as we associate green with the beverage Mountain Dew and one might think of the color blue in relationship with the sky or the ocean, choosing an appropriate base color to represent your site is a good place to start. There are plenty of ways to select a base color; a few ideas include:

  • A color that is naturally associated with the topic of your content, such as the existing color identified with a product or idea or a color representative of the emotion you wish to convey.
  • A color that comes from imagery associated with what your content is about. If you're creating a web site about a given item or product, choose a color that's physically present on that item.
  • Browse web sites that let you look at lots of existing color palettes and images to find inspiration.

When trying to decide upon a base color, you may find that browser extensions that let you select colors from web content can be particularly handy. Some of these are even specifically designed to help with this sort of work. For example, the web site ColorZilla offers an extension (Chrome / Firefox) that offers an eyedropper tool for picking colors from the web. It can also take averages of the colors of pixels in various sized areas or even a selected area of the page.

The advantage to averaging colors can be that often what looks like a solid color is actually a surprisingly varied number of related colors all used in concert, blending to create a desired effect. Picking just one of these pixels can result in getting a color that on its own looks very out of place.

Fleshing out the palette

Once you have decided on your base color, there are plenty of online tools that can help you build out a palette of appropriate colors to use along with your base color by applying color theory to your base color to determine appropriate added colors. Many of these tools also support viewing the colors filtered so you can see what they would look like to people with various forms of color blindness. See Color and accessibility for a brief explanation of why this matters.

A few examples (all free to use as of the time this list was last revised):

When designing your palette, be sure to keep in mind that in addition to the colors these tools typically generate, you'll probably also need to add some core neutral colors such as white (or nearly white), black (or nearly black), and some number of shades of gray.

Usually, you are far better off using the smallest number of colors possible. By using color to accentuate rather than adding color to everything on the page, you keep your content easy to read and the colors you do use have far more impact.

Color theory resources

A full review of color theory is beyond the scope of this article, but there are plenty of articles about color theory available, as well as courses you can find at nearby schools and universities. A couple of useful resources about color theory:

Color Science (Khan Academy in association with Pixar)
An online course which introduces concepts such as what color is, how it's percieved, and how to use colors to express ideas. Presented by Pixar artists and designers.
Color theory on Wikipedia
Wikipedia's entry on color theory, which has a lot of great information from a technical perspective. It's not really a resource for helping you with the color sleection process, but is still full of useful information.

Color and accessibility

There are several ways color can be an accessibility problem. Improper or careless use of color can result in a web site or app that a percentage of your target audience may not be able to use adequately, resulting in lost traffic, lost business, and possibly even a public relations problem. So it's important to consider your use of color carefully.

You should do at least basic research into color blindness. There are several kinds; the most common is red-green color blindness, which causes people to be unable to differentiate between the colors red and green. There are others, too, ranging from inabilities to tell the difference between certain colors to total inability to see color at all.

The most important rule: never use color as the only way to know something. If, for example, you indicate success or failure of an operation by changing the color of a shape from white to green for success and red for failure, users with red-green color-blindness won't be able to use your site properly. Instead, perhaps use both text and color together, so that everyone can understand what's happening.

For more information about color blindness, see the following articles:

Palette design example

Let's consider a quick example of selecting an appropriate color palette for a site. Imagine that you're building a web site for a new game that takes place on the planet Mars. So let's do a Google search for photos of Mars. Lots of good examples of Martian coloration there. We carefully avoid the mockups and the photos from movies. And we decide to use a photo taken by one of the Mars landers humanity has parked on the surface over the last few decades, since the game takes place on the planet's surface. We use a color picker tool to select a sample of the color we choose.

Using an eyedropper tool, we identify a color we like and determine that the color in question is #D79C7A, which is an appropriate rusty orange-red color that's so stereotypical of the Martian surface.

Having selected our base color, we need to build out our palette. We decide to use Paletton to come up with the other colors we need. Upon opening Paletton, we see:

Right after loading Paletton.

Next, we enter our color's hex code (D79C7A) into the "Base RGB" box at the bottom-left corner of the tool:

After entering base color

We now see a monochromatic palette based on the color we picked from the Mars photo. If you need a lot of related colors for some reason, those are likely to be good ones. But what we really want is an accent color. Something that will pop along side the base color. To find that, we click the "add complementary" toggle underneath the menu that lets you select the palette type (currently "Monochromatic"). Paletton computes an appropriate accent color; clicking on the accent color down in the bottom-right corner tells us that this color is #508D7C.

Now with complementary colors included.

If you're unhappy with the color that's proposed to you, you can change the color scheme, to see if you find something you like better. For example, if we don't like the proposed greenish-blue color, we can click the Triad color scheme icon, which presents us with the following:

Triad color scheme selected

That greyish blue in the top-right looks pretty good. Clicking on it, we find that it's #556E8D. That would be used as the accent color, to be used sparingly to make things stand out, such as in headlines or in the highlighting of tabs or other indicators on the site:

Triad color scheme selected

Now we have our base color and our accent. On top of that, we have a few complementary shades of each, just in case we need them for gradients and the like. The colors can then be exported in a number of formats so you can make use of them.

Once you have these colors, you will probably still need to select appropriate neutral colors. Common design practice is to try to find the sweet spot where there's just enough contrast that the text is crisp and readable but not enough contrast to become harsh for the eyes. It's easy to go too far in one way or another so be sure to get feedback on your colors once you've selected them and have examples of them in use available. If the contrast is too low, your text will tend to be washed out by the background, leaving it unreadable, but if your contrast is too high, the user may find your site garish and unpleasant to look at.

Color, backgrounds, contrast, and printing

What looks good on screen may look very different on paper. In addition, ink can be expensive, and if a user is printing your page, they don't necessarily need all the backgrounds and such using up their precious ink when all that matters is the text itself. Most browsers, by default, remove background images when printing documents.

If your background colors and images have been selected carefully and/or are crucial to the usefulness of the content, you can use the CSS color-adjust property to tell the browser that it should not make adjustments to the appearance of content.

The default value of color-adjust, economy, indicates that the browser is allowed to make appearance changes as it deems necessary in order to try to optimize the legibility and/or print economy of the content, given the type of output device the document is being drawn onto.

You can set color-adjust to exact to tell the browser that the element or elements on which you use it have been designed specifically to best work with the colors and images left as they are. With this set, the browser won't tamper with the appearance of the element, and will draw it as indicated by your CSS.

Note: There is no guarantee, though, that color-adjust: exact will result in your CSS being used exactly as given. If the browser provides user preferences to change the output (such as a "don't print backgrounds" checkbox in a print dialog box), that overrides the value of color-adjust.

See also

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