Color and CSS

According to the W3C, a CSS color is either a keyword or a numerical specification.

Color Keywords
Color keywords are a list of words (in English) that correspond to colors on Web pages. There are 16 HTML4 color keywords:
These colors are often called the Netscape Named Colors because the Netscape browser was the first Web browser to support the color keywords.

Colors as Numbers
The CSS2 specification provides for two ways of specifying colors as numbers:
RGB Color Numbers  (the format of an RGB color number is: rgb(red,green,blue);
The red,green,blue are numerical values from 0 to 255 or percentage values from 0% to 100%).
Hexadecimal Color Numbers   (are also RGB - they are just written differently to allow for differences in how browsers handle the CSS. Hexadecimal colors are the same RGB color numbers converted to base-16 and written as one long number - #RRGGBB. Each pair of the hexadecimal triplet is a number from 00 to FF, which corresponds to 0 - 255 in decimal).
RGBA [rgba(red,green,blue,opacity)] color values allows you to define the opacity of the color. The final number is an alphavalue ranging from 0.0 to 1.0.
HSL color values refer to hue-saturation-lightness numerical codes. They were added to solve some specific problems with RGB colors:
RGB is hardware-oriented. Most professional printers are not RGB based, but CMYK.
HSL is not supported by most browsers