Just idea for outlines is from my skins, its not stolen, so don't rate him low for this.
Anyway, about the skins:
Zombie, the old one has better colors, but worst outlines. (Old one is a bit better, overall 3.30)
Elite, its good, though just inverting from left to right handed isn't good since it has that red thing on its arm, it looks really bad now, also, shade is really bad. (overall 2.85)
Sniper, this is the first time I seen it, but, there are also flaws that have nothing to do with colors. Its outlines are really messed up, the sniper/zombie has its head collide with his right hand... (overall 4.15(colors are good.))
Solider, its outlines are great, but colors are bad, too much black outlines in it so you can't shade it well, also, what you already shaded looks really bad. (overall 3.35)