You need to sand the tiles a little, apply two coats of primer, then paint with any type of water- or oil-based paint, then finally top with two coats of clear waterproof varnish. I've done it very successfully using this method, but I believe you can also buy special tile paint.
Indeed this is true, you can buy specialist tile paint from B&Q or Homebase and the like. Not too expensive, but I hope its not a big bathroom, you need to give it a couple of coats, and its touch stuff!!!