NOTE: I do not have iOS and cannot take screenshots, so I make this in the hopes that someone can complete it with pictures and in the correct format.
Update 1.95 (released Sept. 27, 2012) has brought enhanced zombies that have different colors, special abilities, and are a whole lot harder to kill.
Red-tinted zombies have significantly higher health (possibly twice the normal amount), and are very resilient to Frag grenades. It is possible that they are immune to all explosions (excluding the Holy Hand Grenade), but it hasn't been tested. It is possible that they deal more damage, but it hasn't been tested either.
Green-tinted zombies have the same characteristics as Red Zombies, except that they are completely immune to Cryo Grenages instead of Frag.
Every type of zombie is capable of appearing in one of these forms and can be found on any map.