Hi, Although it would seem a bit of an unusual question at first it would make sense to have the prosthetic as similar in colour to the rest of the body.
Whilst most good tanning solutions have DHA as the active ingredient that reacts with the human skin causing the tan, they nearly all also have a guide colour which is basically paint that can wash off. We would suggest you try a patch test as you don't want to discolour the prosthetic in the long run as some tanning solutions do to white salon furniture.