I think it also affects the entire skin color and depends on the ambient temperature.
When Rustem was found, he was frozen and later there were of course higher temperatures in the room and so I think that the color of the skin has changed again because of it.
Corpse stains form where the body has contact with the ground, for example, I mean. Because Rustem was lying faced in the snow, towards the ground, I mean that the face color was so dark because of it. So the face color should have returned to normal after he was lying on his back in the morgue.