Hexadecimal (often
abbreviated hex) is a 16 numeral system, usually
written using the symbols 0-9 and A-F. It is a useful
system in computers because there is an easy mapping from
four bits to a single hex digit. Thus one can represent
every byte as two consecutive hexadecimal digits. Colors
in HTML are specifyed using hexadecimal numbers.
Most graphic design programs (photoshop, photopaint,
paintshop pro etc.) offer to handle colors in a RGB (Red,
Green, Blue) color system. To define a certain color you
simply enter the amount of each of the three colors Red,
Green and Blue.
Another way to convert a decimal value into a
hexadecimal value is using the Windows standard calculator
(If you don't see the hex-option in the calculator, click
the view-menu and choose scientific). Enter a value, click
the Hex-field and the calculator will convert the number
to hexadecimal.