I've never tried this, and have no idea how flexible the paint would be. But I had good luck with using one of those little bottles of acrylic patio furniture paint on my bathroom sink when I rented a trailer that was older than me. I wonder how fabric paint would do on a canvas/nylon fabric shower curtain?

I'd try acrylic on a fabric type curtain. Acrylic, when thinned a bit, works wonderfully on fabric and is very flexible. Once it's dry it doesn't smell at all and is impermeable. It would work on the plastic curtains and i don't think it would make any smell when heated or anything, but i don't think it would stick well enough under hot water to stay. I use hot water to clean up my palettes and it makes the dried acrylic loosen enough to slide off. It might be ok on the outside of the plastic, but i wouldn't try to wash it in a machine. The fabric will give it something to grab on to, which is why it should work better. There are special things you can add to it to make it more fabric friendly, but i have used acrylic, very thin like watercolor all the way through fairly thick on fabric just plain and it's held up just fine, even through many washings. I don't think you'd need to worry about sealing it. Esp if you thinned it enough with water that it soaked into the fabric, it's not going anywhere.

That could be the way to go. I've been looking for a curtain I like but nothing appeals to me so I figured I would do my own.