#border-bottom-color

Like border-color, but for the bottom border only.

border-bottom-color: transparent;

Applies a transparent color to the bottom border. The bottom border will still take up the space defined by the border-width value.

border-bottom-color: red;

You can use one of the 140+ color names.

border-bottom-color: #05ffb0;

You can use hexadecimal color codes, rgb(), rgba(), hsl(), hsla()...