The tile is blue and white and are each about 3x3 inches wide?Can you lay new tile over the top or paint it?
Using TSP (trisodiumphosphate), thoroughly clean the tile surface. The TSP will clean and degrease the tiles. At Home Depot there is paint that you can buy that is designed specifically for painting over ceramic tile. I've used it and it does work, but make sure you open some windows and have some cross ventilation going as it smells very strong and the fumes can cause problems. It is supposed to be self leveling, but being as you are working on a vertical surface, it's best to use thin coats of the paint. If you use any other type of paint, it will come off when you go to clean it.
you can do either if you paint you need to prime it 1st and use 100% acrylic paint
Screw 1/4 plywood over the tile and apply the mosaic over that....
