HTML web colors are colors used in displaying web pages, and the approach for characterizing and specifying those colors. Colors may be specified as an RGB triplet or in hexadecimal format (a hex triplet). They may also be specified according to their common English names in some cases. Often a color tool or other graphics software is used to generate color values. In some uses, hexadecimal color codes are specified with notation using a leading number sign (#).
A color is specified according to the intensity of its red, green and blue components, each represented by eight bits. Thus, there are 24 bits used to specify a web color, and 16,777,216 colors that may be so selected. When you combine the 3 colors, you get a specific variant. It is the same concept as mixing paint together.
The first versions of Mosaic and Netscape Navigator used the X11 color names as the basis for their color lists, as both started as X Window System applications.