Hexadecimal numbers or "hex" numbers are a base-16 numbering system used to define colors on Web pages. A hex number is written from 0-9 and then A-F.
Web colors are written as three sets of hex pairs. The first set represents the red hue, the second the green hue, and the third the blue. White is written as ff ff ff or
#ffffff while black is 00 00 00 or
When using hexadecimal colors in web design you are using the RGB color model. This is a color model that describes projected colorscolors as they are seen directly by our eyes, not reflected off of any surface. Computer displays work with color this way, and so the RGB color model is most appropriate for web designs. If your pages are going to be printed, you might consider switching to a reflected color model such as CMYK for your print style sheets.