13 graphics does NOT use 24-bit colour. If you had read what I had said, only 6 of the eight bits of each colour value are used. This means that you can only pick your palette from 2^18 colours (262,144 colours), not 16,777,216 like in 24-bit modes. Gotta love these old standards...
e.g. if you set a register to RGB 215, 34, 167:-
Code:
11010101 00100010 10100111
||||||XX ||||||XX ||||||XX
Only the the bits with pipes underneath them are actually used, the bits with Xs underneath them aren't used. This results in the actual value being RGB 212, 32, 164.