Actually, tortoises don't have skin under their shells. If you look closely around a tortoise's head, legs, and tail you will notice that the skin attaches directly to the shell. In fact, the shell is an extension of the tortoise's ribs and spine. So the answer is that the color under a tortoise's shell is probably red (blood). I hope this answers your question. If not, feel free to reply to this posting or directly on my blog at TheGreekTortoiseGuild.blogspot.com