A buddy of of mine(alex) has reminded me if they are green, even if wearing PH armor, technically its the period of The Great Scouring.
Below excerpt taken from 40k lexicanum:
"The Great Scouring was the Imperium's counter-offensive against the rebel armies of Horus, following the defeat of the Horus Heresy in c.M311. Horus's campaign to strike towards Terra to achieve a quick and decisive victory had ultimately been defeated. The rebels were broken and in retreat.
Before actually being confined within the Golden Throne, the Emperor had pronounced judgment on the rebels: they were to be driven into the hellish region of the Eye of Terror, which would hold them for all eternity. All records and memory of the Traitor Legions were to be expunged from the Imperial archives. Worlds such as Isstvan V and Davin would be scoured and the Traitors' associated troops were to be destroyed or driven into the Eye. It would be as if the Traitor Legions never existed."