HEXADECIMAL (or HEX for short) is a way of representing a number from 0 to 15 using the letters A to F for the numbers 10 to 15.
It saves space. Computers love numbers that are multiples of "2" (because binary) and counting up to 10 takes up the same amount of space as counting up to 16 so you may as well.
But when you display it on screen or print it out it takes less space to write "F" than it does "15" and "FF" than "255".
This depends on how bloody marvellous your screen is, but MOST screens can cope with a mix of 256 shades of red, 256 green and 256 blue (which astonishingly is more than 16,000,000 colours)
This is most easily represented as a HEX value with 2 characters for red, 2 for blue, 2 for green.
Also note we put a hash in front of HEX values, e.g. #ff = 255
|#ff0000||is as white as you can get|
|#ff0000||is as red as you can get|
|#00ff00||is as green as you can get|
|#0000ff||is as blue as you can get|
|#000000||is blacker than the night|