This is less of a formal suggestion and more of a question for mods regarding tags similar to red_and_white; Since monochrome literally means one colour, shouldn't posts that use red/grey or black be tagged the same as posts that use red/white, as white/black/grey are not colours?

I would have just made a formal suggestion for a new alias, except multiple suggestions (10) would rely on the same concept

Hmm, I'd want to agree and say +1 for doing all those,

but there are bound to be some exceptions which may break it

eg some spot color/restricted palette posts

But I can't offhandedly recall any notable discussions about such

if neither of those or similar cases would contradict it, then +1 from me

(there are also a bunch of other art tags/scenarios I'm probably forgetting about)

Nah. This was been covered several times on the forum, so I'll keep this short.
Check black_and_white for the difference. The other <color>_and_white tags follow the same format: they're a bit more specific than just monochrome_<color>. Although, come to think about it, those should be implicated to monochrome.