I suspect it's some kind of water based paint, so I think eventually the paint would disappear and the plant would have normal growth. I'm not too sure anyone who bought them would want the natural plant though.
Before I knew I could grow anything/keep anything alive, I bought a purple one. After having it for almost 3 years it's beautiful with the beautiful natural growth and it needs a bigger pot again. I've learned a lot from that cactus that was painted that awful purple color. You can still see the purple but the natural unpainted new growth mostly covers the purple. My sister is doing the same thing. Her first one died of overwatering, she showed me after... poor baby she named it aqua. She did a burial ceremony for her before giving her back to the earth and got another painted one that looks very well taken care of and loved, she's now excited for it to look like mine- at 24 years old she named this one greenie.
Now I have more plants (never again painted) and grow a vegetable garden all because of that stupidly painted cactus which I cherish because she taught me so much about living things and myself when I was at my lowest. So that's one of the reason people would buy them.
