column-rule-color

/* Значения <color>  */
column-rule-color: red;
column-rule-color: rgb(192, 56, 78);
column-rule-color: transparent;
column-rule-color: hsla(0, 100%, 50%, 0.6);

/* Глобальные значения */
column-rule-color: inherit;
column-rule-color: initial;
column-rule-color: unset;

<p>This is a bunch of text split into three columns.
   The `column-rule-color` property is used to change
   the color of the line that is drawn between columns.
   Don't you think that's wonderful?</p>

p {
  column-count: 3;
  column-rule-style: solid;
  column-rule-color: blue;
}