All orcs were once brown and native to draenor.
According to The Rise of the Horde, orcs gradually started turning green through exposure to fel magic (similar to how the blood elves eyes turned green from fel energy).
When Kil’Jaeden contacted Ner’zhul and started the war with the Draenei, orcs were brown and still practiced shamanism. The elements grew angry with the orcs for killing the Draenei and started to ignore them and their shamans. Without the elements, the shamans were powerless and converted into warlocks with the help of Kil’Jaeden.
With the introduction of fel magic to their society, all orcs then moved from their ancestral lands to the vicinity of the Hellfire Citadel to become a stronger army. There, the fel magic combined with the fel energy of the demon blood was so concentrated that their skin gradually turned green over time. The blood didn’t do it by itself, it was the overall atmosphere of fel energy.
After they were green, the green skin became genetic and, thus, all later generations of orcs were born green regardless of their exposure to fel magic.
Durotan and Draka (in the original timeline, not this confusing Warlords of Draenor stuff) were both born brown and turned green when the moved to the hellfire peninsula despite never drinking the demon blood and that is why Thrall is green.
Garrosh and Aggra are both still brown because they were members of a clan that never moved to the hellfire peninsula and, thus, were never exposed to the fel magic that was concentrated there.
Warlords of Draenor orcs (like the Durotan and Draka pictured above) are still brown because presumably Garrosh altered the timeline not just before they drank the blood, but before they moved to Hellfire peninsula and the fel magic could effect their skin.
Its still more complicated than that but thats about as abridged as I can make it.
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