Margin color

border: 1px solid #BORDERCOLOR;

I can designate BORDER COLOR like the above.
How can I designate MARGIN COLOR?

Margins don’t have a color. They just push elements apart. You could use a wide border instead, or you can have a background color showing in the padding area.

The margin colour will be whatever colour is assigned to the container that the elements are in. So simply set the required colour on the parent element.