Coated with what?
If they just ate a lollipop that could make it purple.
A white film that rubs off leaving an erythematous base in an otherwise sick person is most likely Candida.
If its darker brown/black in the middle of the tongue, doesn't rub off, and is mushy/hairy (not maculo-papular) it's likely Hairy Tongue. I think this is due to elongated filiform papillae (or is it fungiform?) when they fail to slough normally. I can't remember why it happens, but it's mainly in people who are bedridden or have other chronic illnesses, and also people who use a lot of antacids for some reason.
Also on the list is Lichen Planus, geographic tongue, leukoplakia, infections (candida,syphilis). I'm sure the list could go on and on unless you give us more info.