How screens diplay colors

Technically speaking, colors are the way our brain, by use of our eyes, interprets electromagnetic radiation of a wavelenght between 350 and 750 nanometers.
The different wavelengths are seen as different colors, as in the spectrum below. You have probably seen a spectrum like this before. Electromagnetic radiation with a wavelength of over 750 nm is called infra-red, and radiation under 350 nm is ultra-violet.

As discussed in my section about, Color Theory, a computer monitor is made up of thousands of tiny little red, green and blue dots which are grouped three and three. These little dots are close together-- so close that we don't really see them as dots, but rather our eyes mix them up to form one homogenous color.
All colors in the spectrum can be generated with this system of dots. For instace, if the red and green dots are shining at 100% their strength, while the blue not lit at all, you will see the result as a pure yellow color. Each of the three dots can have any value from 0 to 255, which means that the total number of possible colors is 256*256*256 = 16,777,216.

Any color can be represented by a specific combination of three numbers. The yellow mentioned before would have the number 255, 255, 0. The first number represents the red dot, the second represents the green dot, and the last represents the blue. Therefore, a pure blue color would then be 0, 0, 255. The lower the number, the less that color dot is lighted. For instance, by lowering the number for the blue dot-- to the color identified as 0, 0, 100-- you can produce a darker blue. White is produced by the combination of the highest amount of all three colors, so the code for white is 255, 255, 255. Black, on the other hand, is produced by a lack of other colors, so the code for black is 0, 0, 0.

From Sitepoint