Not sure if you're aware of this, I just learned it in the last few weeks since I began keeping hermit crabs (have 4 now) but painted shells are actually dangerous to hermit crabs.
Such as: *They can end up eating paint pieces that have chipped off which is often poisonous.
*Paint can begin to get damp in the humidity of their crabitat, seep through their shell and get on them, sometimes sticking them to the shell so they cannot change to a new one. I saw a lady on a hermit crab Facebook group who had to free one from just such a situation a day or two ago.
I'd recommend this more as a craft item or something rather than for a hermit crab. :)