No time to finish, but a partial...
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I think the history is something like this.
Start with 8 colours because 8 is a small power of 2. These are simple: bit 0 for red, bit 1 for green, bit 2 for blue, but 000 means grey rather than black because you're only controlling the foreground and not the background.
Then add another 8 colours. Actually, in the image here it looks as if 8-15 may be the ...