The most important step is the prep. Make sure they are perfectly clean and then use a maximum adhesion primer like Bondz from Zinsser.
Here are some ideas for your tiles
Rustoleum or Homax tub and tile paint kits.
Hello, I have personally not done this exciting refurb project —but here’s a link that might be helpful.
Hi! I have used Rustoleum Tub and Tile and it worked great! Good luck!
How to Refinish My Kitchen Sink DIY | Hometalk
There are some awesome kits out there. I would ask at the paint store though, the pros know which really work best.
You just need to use an oil based paint and prep propely. IF you go to home depot or lowes they can help you find the right paint to go over tile
This may help: http://www.abeautifulmess.com/2015/09/how-to-paint-a-tile-backsplash.html?crlt.pid=camp.KLP7nta2XMMC
Try this: https://www.aratariathome.com/blog/painted-tile-tutorial-entry-way-makeover-part-2
There is special kind of paint at the Home Depot for you to be able to paint them
Here's a post that might help - https://homeguides.sfgate.com/way-paint-subway-tiles-42943.html
Subway tiles are pretty desirable these days but if you really want to paint them, there are tile paints available. You can see how I painted over my ceramic tile floor for less here: https://www.flippingtheflip.com/2019/03/how-to-stencil-paint-ceramic-tile-floor.html