See also RGB Color Chart.
To investigate further, look at How to Think Hexadecimally.
Base 16 has 16 different digits. One half byte or one nybble or four bits has SIXTEEN different possible values, from 0000, 0001, 0010, 0011, 0100, ... to 1110, 1111. In base 16 that would be from 0, 1, 2, 3, 4, ... to E, F. In base 10 0, 1, 2, 3, 4, ... to 14, 15.More RGB background pages including Dougie's Color Picker.