After Istvan the Raven Guard got pure gene-seed from one of Big E old projects if memory doesn't fail me, that then got corrupted by a sleeper agent Alpha Legionary with demoic blood. Stealing the pure sample for the Alpha legion/ Horus. However Omegon leading the operation stole the pure sample from the Alpha Legion exchanging it with a flawed sample.
So they could only give the imperfect corrupted sample to Horus.

Typical Alpha legion shenanigans all in all.

Ye the old school colours were a tad more towards turquois. The current way to do them with incubii darkness into sybarite and cabalite green just doesn't feel right.

Lovely paint job on the base and Archon, may I ask how you did the armour looks similar to the old Black Heart artwork.