The chart below illustrates the RGB values for specific colors.  Think of the charaters as a code: the first 2 are for Red, the second 2 are for Green, and the last 2 are for Blue

Each set of 2 can range from 00 to FF, 00 for the least amount of that color element and FF for the maximium amount of that color element. For example look at the color assigned to 000000 which is black, all values are set to 00, or the minimium - resulting in black.

Then look at white which is set to the maximium for each element. Black is considered no colors, white is considered every color, or full of color. By mixing the values, a nearly unlimited number of colors can be generated.