as far as I know, Green orcs
WERE tainted by demonic influence.
Red orcs are that way because
they are still under the influence of Demonic corruption.
I could be wrong, but I think that's the case.
---------- Post added 2013-06-17 at 05:21 AM ----------
From WoWpedia:
The fel orcs possibly chaos orcs[2] are the ultimate culmination of the Burning Legion's control over the orcs.
An orc becomes a fel orc after consuming the blood of a pit lord. Then, a heavy blood haze falls over the orc, forcing him into a crazed bloodlust.
The most prominent side effect of this blood haze is a deep red discoloration of the flesh and insane desire for battle and violence.
---------- Post added 2013-06-17 at 05:22 AM ----------
and more
Physiology
The standard fel orc has red skin, and a ridge of long sharp spines coming out of its back. Bony horns grow from its arms and hands. Its eyes are bloodshot and glow red, and six long and thin fangs grow out of its mouth. Its ears are stubbier than orc ears, coming to a blunt point. Most fel orcs have lived for years in Outland with the corruption, and this has led to the greater mutations.
Relation to tainted orcs
See also: Chaos orc
Tainted orcs is the RPG's term for green orcs, that is, the orcs that were corrupted by the Burning Legion's influence on Draenor. These orcs appear to be different from fel orcs, as they don't posses the physical attributes of a fel orc.