The first two digits of the number represent the red value of the color, the next two represent the green value, and the final two represent the blue value. All of the colors on our computer are represented using different combinations of these values.
So what's with the letters? Although as humans we usually work in a base 10 system of counting, when dealing with colors we work in a base 16 system. That means that 0-9 are represented the same, however 10-15 are represented as A-F before adding a second digit to a number.
