I successfully kept a very beautiful true apple snail (the kind with the really long air siphon) in a 10 gal until he got about 2/3 the size of my fist then I had to move him to a 29 gal where he promptly made a big salad out of my plants X) the little rascal! Anyway, he had dark grey to black flesh and his shell was variations of gold to dark brown. You're lucky if you can find a apple snail like that these days though, most of what you'll find are mystery snails but there are some dark ones (black mystery snails for example) that are very cute.