Why are there brown orcs such as Warchief Garrosh Hellscream? Why are there only a few of them scattered throughout [World of] Warcraft's Azeroth and Outland?
All original orcs were brown-skinned; from bark-like brown to reddish-brown. However, their bodies react when exposed to warlock magic; though the nature of this change is somewhat different in other fel-touched races. The nature of orcish fel magic means that all nearby orcs, including those who avoid warlock magic, take on a green pigmentation and gain body mass (though why the Mag'har are much bigger than Durotar orcs in World of Warcraft is unknown). This color change appears to be genetic as Thrall, who had little direct exposure to warlock magic until recently, has had green skin from birth.
As this corruption progresses, the orc's eyes may become flaming orbs of red or green and their skin will soon change from green to scarlet, transforming them into fel orcs. Through certain rituals, this state is reversible. However, should the orc continue to drink large amounts of demonic blood, they enter the final irreversible stage of transformation and mutate horrifically; growing additional fangs and erupting with horns from their backs, arms and hands.
Currently, orcs of the Horde have a wide variety of "natural" pigments, ranging between shades of light green, dark drab olive, brown to grayish brown.