tongues should have texture to them, and the very back of the tongue with large bumps is not used in tongue diagnosis, only the easy to see front part is.
When you're bowel cleansing it's normal to see some ugly tongue stuff as things get stirred up. If it's black, that means your intestines have too much Vata - gas, coldness, so you need pungent/warming therapy. Avoid raw foods, iced drinks. See the gas-bloating-pungent therapy post for ideas as to how to fix your foods so this doesn't happen. It will help white coating too.
Anytime you see yellow, that means bile. Too much bile in the small intestine. Cleansing Carrot Salad is great for that.