Just so there are no toes being stepped on, are there any particular bits of etiquette or rules of thumb on posting a colored version of an existing, uncolored post?

I think it'd be safe if you got the original creator's permission and possibly the commissioner of whatever post you're attempting to color.

Also, tag yourself as artist:name_(colorist) and be sure to tag the original artist as well.

While it's probably the safest to get an artist's consent to colorize their work we don't actually require it to my knowledge. Though the caveat applies that if the original artist or commissioner comes along and goes "Yeah I'm not okay with this" then it's getting deleted.