The carbon must be represented by the color black or white.
Ideally, represent the elements with the color closest to their actual color. In all its allotropic forms, carbon is in a solid physical state, but other physical characteristics vary from structure to structure. The color of the carbon varies with the organization of its molecules, in which case the diamond's carbon is transparent and the graphite carbon is black. For this reason the ideal would be to represent carbon with a black or white color.