> 16-bit machines (Apple Mac, Amiga, Atari ST)

(small nitpick) These platforms all began with Motorola 68000 which has a 32-bit ISA.

... with 16bit bus.

Which is why we call the machines 16bit; they're built around 16bit buses.

Three

That seems similar to the "perwinkle blue" that was used as the default background in Apple IIGS ProDOS16 / GS/OS. In that case it was done with pairing white pixels with blue pixels.