... And assuming all you want to do is change the 'color' of the wall. If this is the case, you need to use a Primer+Sealer first to prevent the DARK color from 'bleeding through' to your new top coat. It wouldn't hurt to put on 2 coats of primer+sealer, then 2 top coats after that. DO read the Instructions BEFORE you leave the store. Make sure you understand all the preparation and instruction information. Stick with the same 'line' of primers/sealers/paints.
And, go to your local PAINT - RETAIL Store (found in the Yellow Pages under PAINT - RETAIL.) None of the 'regulars' say 'nice' things about the paints found in the BIG BOX Home Center Stores, although we shop for pretty much everything else there. You'll spend more $$$ at the RETAIL Paint Store but, hey, you get what you paint, er pay, for. If you want to tell me/us why you want to remove paint from drywall, that would be an interesting follow-up. Maybe one of the kids decided the paint book wasn't any fun any more?! And what color is the painted wall? And what color is the paint the 'kid' put on the wall???