This phrase makes little sense by itself. You probably heard it as part of a longer expression such as the one srgause mentioned. In general, the phrase "your ass" often appears in threatening or insulting expressions, meaning "you" or "your body". There's a linguistic term for this phenomenon where "a part represents the whole", and, if I manage to remember, I'll post it in an edit. Anyway, as an example of the proper (if one can really use that word in this situation) usage of "your ass", in this sense of the expression, here is R. Lee Ermy's famous line from the movie "Full Metal Jacket": "God existed before the Marine Corps; so, you can give your heart to Jesus, but your ass belongs to the Corps!"

Hi, it could be similar to "paint your ears" When you say something that you know the other person just wants to hear. It may not be the truth but it makes them feel better. It's kind of sugar-coating the subject. Or have you confused it with "paint the town red?" That means to go out and have a good time. In any case using the word "ass" or your bottom is impolite and should only be used amongst friends.

Where did you hear that? I've heard people say "I'll paint your ass red", which means they will beat them until they turn red. It's not a nice term and extreme slang so I would avoid using that phrase.