@WhyNotHugo please advise if you think it's better to support more terminals or to use 256-bit colors. Theoretically we can do both (have 256-bit colors but have a fallback for old terminals) but it'd be messy

I'd stick to documenting it as a requirement. Not sure if the command is worth it, but I wouldn't be against a check command that checks this (I say check, because we might want to extend it to other checks that come up in future).

I'm open to PRs for supporting terminals that don't support 24-bit colours though, if somebody cares enough, but I wouldn't worry about it ourselves.