The hardware colour index (defined by the I/O write: A15="0", A14="1", D7="0", D6="1" D3-D0 define hardware colour index), is used as a look-up into the ROM.
Each colour-element (R, G or B) is defined using 2 bits and has the potential to define a total palette of 64 colours. The colour-rom only uses 3 of the 4 possible settings for each colour-element. Looking at the schematic, and it the potential is unknown if not realised, the two of signals for each colour use the settings give same resistors before being combined. This means that the bit combinations 10 and 01 produce the same result intensity.So this means only 27 colours are possible.
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