If it's a girl butt, a lot of people use a heart pan, then cut the point of the heart off and use the rest of the heart upside down to be the butt cheeks. I made a butt before with two cakes baked in a 2 quart glass mixing bowl and another square cut up to be the rest. It was my first naughty cake and wasn't a very feminine butt, but it was still pretty effective.
To make flesh color, Wilton makes a flesh shade, or you can use ivory. You can add a bit of peach if you want a pale pink skin tone, or add a bit of brown for a more tanned look. If you look in my photos, the nude torso was done with wilton's ivory and the butt in the black thong was done with Wilton's flesh color. The boobs in the white shirt were done with a mix of wilton's ivory and peach colors, then painted with ivory mixed with vodka.
The only thing about painting flesh color is that it's difficult to get it done without any streaks, unless you have an airbrush (I don't).
You could get a food coloring marker and draw a tramp stamp on 
Just practice before you actually draw on the cake, fondant is a bit more wiggly than paper.
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