Anyone who ever builds a website in HTML will benefit from having an understanding of hexidecimal notation, as this is the format in which HTML denotes color.
Colors in HTML are written in the format #------, with a hashtag (#) followed by three pairs of two-digit hexadecimal numbers (00-FF). The first two digits determine the amount of Red in the color used; the second two determine the amount of Green; and the third two determine the amount of Blue used. This system is commonly referred to as RGB color coding.
Note that the three primary colors used are NOT red, yellow, and blue, because this color system is additive rather than subtractive, thus the subractive primary color yellow is replaced by the additive primary color green. Read more about additive and subractive color systems here: http://www.worqx.com/color/color_systems.htm
RGB color coding can represent over 16 million colors (256 x 256 x 256) or (FF x FF x FF) in hexidecimal.