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:-
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.
11010101 00100010 10100111
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