Purple? They turn purple when they are mad, so I'd assume it would be the same if they were choked. Our skin turns blue for a reason. The tissue closest to the surface of the skin lacks oxygen. This brings out a blue coloration because of the pigment in the skin. It takes little to no thought to realize that though. I could go more into detail about all of that, but I'll save that information. Anyway the color blue is harder to detect on skin of a darker pigmentation. I can go into two directions or more with this question. The skin could turn purple like I said and it probably would if it was made of the same substance as our skin. It's not our skin though it's smurf skin. The second color it may turn is white. The skin has blue pigment already, so it would kind of make sense if it lost coloration. It could appear gray and not white in come cases. One other way I think about it is nothing happening to it because of the fact that they are not real and they don't have oxygen in their blood. Perhaps they don't even have blood at all. You can choose an option you like the best. Whatever it'll be is up to you. Pick something you like the best. ;)
Or, perhaps a bag of "magic mushrooms"?