The turtles shell needs exposure to air. It must be able to dry out when it is basking. Also it needs to have access to UVB light while basking to keep the shell strong (in combination to a calcium supplement such as cuddlebone) If you paint the shell you will block out the needed light to absorb the calcium to keep them healthy and their shell strong. Also think about it.... if you used paint that isn't toxic, it would be a water based paint, turtle goes into the water, paint washes off in the water, now you have to change the water immediatly so as not to poison your turtle. Do not paint your turtle! If you want a pet to paint, get a hermit crab. You can buy shells with safe paint, and paint to your hearts content! (Not my work)