This question assumes that a smurf requires oxygen to survive, which is a valid assumption considering smurfs fear drowning as much as any normal air-breathing creature does. Furthermore, if smurfs require oxygen, then they must have some kind oxygen-transport molecule in their bloodstream. In vertebrates, this molecule is heme with an iron atom at its center and is what gives blood its red color. Since smurfs appear to be vertebrates, we can assume they also have heme in their blood. Now when a naked-skinned vertebrate is choked, namely humans, their faces turn red because surface capillaries in the skin dilate and blood rushes in. Assuming the same thing happens to smurfs, their faces would turn purple.