The most well known color model is the RGB (Red-Green-Blue) one. All the colors can be obtained by blending various amounts of each of these 3 basic colors. This system is used by TV cathode ray tubes and computer monitors. Each pixel is made of 3 small adjacent phosphors emitting respectively red, green and blue lights when excited by the electron beam. This model is said to be additive because each color is obtained by adding the lights emitted by each phosphor.

For example, equal values of red + green make yellow. In computer images, the best that can be done is to code the intensity of each color on one byte. Therefore the emission of each phosphor can have 256 distinct levels (from 0, for black, to 255 which is the maximum of emission for this color). Each pixel being made of 3 phosphors, this technique allows the coding of 256³ = 16,777,216 colors. This mode is usually named: 16 millions of colors, true color ,or 24 bit color mode.
   

You must understand that an image can’t have more colors than the number of pixels (480000 for a 800x600 pixels image). You must also understand that if you only use one color to create a monochrome image (for example pure blue), you can have only 256 shades. It is the same if you create a black and white image.
   

Everybody knows that the eye has not the same sensibility for all the colors. It is why blue = 255 appears several times darker than green = 255. Indeed pure blue is not very bright and a pure blue figure on a black background looks poorly contrasted. But the most luminous yellow that you can obtain is made of red = 255 + green = 255. Therefore, mixed colors can be more luminous that pure ones.
   

However, this technique gives the best possible results in almost all the usual cases. It can give very vibrant as well as pastel-like colors.