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Your choices of numbers and letters to combine include:0,1,2,3,4,5,6,7,8,9,a,b, c,d,e,f. They can be used in any combination. Here is a list of alphanumeric strings, coloured as they create (note that there are 16,777,216 possibilities, so I'l only list the major ones):

Red + 00 + 00
00 + Green + 00
00 + 00 + Blue
Red + Green + 00
Red + 00 + Blue
00 + Green + Blue
Red + Green + Blue