How can a box border colour be specified?

The border-color shorthand CSS property sets the color of an element's border.

/* <color> values */
border-color: red;

/* top and bottom | left and right */
border-color: red #f015ca;

/* top | left and right | bottom */
border-color: red rgb(240,30,50,.7) green;

/* top | right | bottom | left */
border-color: red yellow green blue;

/* Global values */
border-color: inherit;
border-color: initial;
border-color: unset;

This property is a shorthand for the following CSS properties:

border-bottom-color
border-left-color
border-right-color
border-top-color

Source: https://developer.mozilla.org/en-US/docs/Web/CSS/border-color