We have different number systems for our calculations. Most popular one is decimal system which is of base 10. In digital mathematics we use binary system of base 2. Similarly we use octal system with base 8. Hexadecimal system uses 16 as base. For colour specifications in our HTML codes we use Hex numbers. For example to display blue background color we have to use hex code #0000FF. This is same as Red =0 , Green =0 and blue =255 in decimal system.
Here is table to give you idea on how the number system changes with different base values.