I know it says this one was resolved. However, I successfully painted toilet floor tiles. I can't stress enough how important the prep is. Use the tile cleaner spray. Undercoat or prime with whitknight primer. I'll add a photo of this. Also to my knowledge currently the only paint that works on floor tiles is rustoleum milk paint. There is a sealer, however I was not game to use this in case it made the flooring slippery. After my 2 coats of milk paint I was so happy with the result. To be honest, even if it does eventually chip it will still look better than brown 70's mosaic tiles. Also, I still have half a can of paint so could always do another coat. Almost forgot, drying time is ultra important. T
he longer you leave it the better. It seems to harden or something. So, I recommend painting the floor before you go away for a few days.
Yes, painting horizontal surfaces isn’t normally the best long term solution. Vertical surfaces (cupboard doors and wall tiles) seems to get the best result. We had a hideous burgundy colour floor tile in our main bathroom and a navy blue in the ensuite and decided to tile over tile. This is the second house where we have done this and the result is perfect.