Tries to pick the best of a set of 16 "CGA color palette" colors
by first mapping each of the RGB components to 0x00, 0x55, 0xAA,
or 0xFF (so 64 possible colors), then finding which of the 16 CGA
colors best matches it.
Also, don't accept the same color as the "background"/"clear" of
"ASCII Typewriter" (white) or "ASCII Computer" (black), to avoid
painting solid colors; pick the reverse color. (In other words,
both black and white choice draw white computer text on black
background, for example.)
