7.2. Color in CSS
Elements that can be displayed in colors defined through style rules are:
I've listed that last one for the sake of completeness. Outlines are not widely supported in any significant percentage of the browsers available today, so we won't spend time discussing them here. Refer to the outline property in Appendix B, if you're curious.
7.2.1. How to Specify Colors
When you specify a color for any CSS property that accepts color values, you have a choice of several methods:
descriptive color names
system color names
RGB decimal values
RGB hexadecimal values (including a three-character shorthand)
RGB percentage values
The most human-readable way to specify colors in HTML is to use key words ...