Actually, I have a real tat, a very small flower, so small it fits under my wristwatch. But there is something repugnant to me about painting a cactus.
Oddly, my opinion about painting does not extend to all plants. One year I painted the outside of some dried gourds and made bird houses out of them and I felt perfectly OK with that.
Since tats were brought up, I realize this is straying a bit off topic but as a retired nurse, I would like to warn against the recent practice of scleral tats. Just a bad, bad idea. Please don't do it.