painting tiles

Painting tiles?
When we moved into our house the tiles in the bathroom had already been painted with tile paint (horrible old lady blue colour). I now want to paint over them with a white tile paint (until we can afford to get it done properly). Would I be able to do this and what would I need to do to prepare the tiles for painting.?
Thanks very much for all your help.
Tile paint is very expensive and looks shabby…I would rather wait till you can afford to tile xxx