Well, Archimonde is a Man'ari Eredar too and he is an example that they don't have to be red. In Warcraft III they can be all colors between Red, purple & Blue. The creator of this picture (Not me) used him as the Man'ari Model and did nothing wrong with that, because he is indeed a Man'ari Eredar...
Hmmm... didn't know that Wrathguard-thing :-/ Aren't the Wrathguards just melee Demons like many others without any special background-story? And what is the difference between Demonic Eredar and Man'ari Eredar?
However, don't blame me, it's taken from wowwiki...! :-/