Something that always annoys me about Lord Cypher art, especially the GW Lore images is that he’s always wearing Dark Angels green armour. He hails (as do all the Fallen) from the time of the Horus Heresy, at the time of the schism the Dark Angels Legion wore black armour with additional white and red trim.
It was only after the death of Horus and the entombing of the Emperor on the Golden Throne that they changed their Legion and subsequently Chapter colours to green. That also changed their Chapter symbol from a vertical pointed sword with a fracture to the downwards pointed sword flanked by wings.
HE WEARS BLACK!!
I had originally thought that Cypher should be wearing black as well since he’s survived since the time of the Horus Heresy, but I’ve found that there are a couple different reasons Cypher should be wearing green. One being that when Caliban originally fell to Chaos under Luthers rule, there were many Dark Angels still on Caliban that did not agree with his teachings, and when fighting broke out, they repainted their armor green, to show that they did not follow him. If Cypher is indeed a Loyalist and is trying to absolve the Dark angels of their sin, then it would have made sense that he changed his armor to green with the heresy era Loyalists. Another theory brought up in tales of Cypher is that since “cypher” is a title and not a name, green armor could have simply been the color of his post. FInally if cypher is in fact carrying the dark sword of Luther, and plots to kill the Emperor, it would be a clever rues for him to have green armor, and spread suspicion about the dark angels. I personally prefer the look of Cypher with his guns crossed and his armor super dark and and the glow of his plasma pistol bathing him in blue green light as shown above. Its like his armor is the deepest of greens, almost iridescent.
I can see these arguments having merit, although I’m of the mind that all the Lutherites are loyal and it’s the Lion and his followers who are the traitors. From the novels the Lord Cypher seems to be the one pushing Luther towards breaking from the Lion’s command, and leading Astellan into killing off the “Lionites” on Caliban.
As to changing the colours to blend in, that might work but as most people in the Imperium, including the Imperial Guard, most of them wouldn’t know who Chapter he was for anyway and the icons on his armour would take weeks or months to trace due to hidden information and sequestered materials from the Horus Heresy. The Fall is one of the longest running and biggest cover-ups in the Imperium.
I think what colour he wears really depends on your stance on the Fall and the DA in general. Personally I have him painted in black with pure white robes, a throw back to his position guarding the Orders rituals and secrets from old Caliban. In my mind the Lutherites are the loyalists and retained the colours that the Emperor gave them at the beginning of the Great Crusade, it was the traitors and the followers of the Lion who changed their colours and their allegiance, hiding in plain sight for millennia.