How can a box border colour be specified?

/* <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;