During one of Games Workshop's open days, they had a very impressive diorama of the Siege of the Imperial Palace:
You will notice that the Chaos Marines in the foreground are in the Black Legion livery.
However, according to the Index Astartes, Horus' legion was still the Sons of Horus in the repainted armour after the Ullanor crusade but before Abaddon renamed them the Black Legion:
Abaddon knew that the memory of the Warmaster shackled his Legion to the failures of the past, so his first edicts renounced the name of Horus and the ancient title of the Legion. Taking their last surviving battle barge, he led them in a lightning raid that destroyed the Warmaster's body and the whole cloning laboratory complex. For this action and in every subsequent sighting, each Space Marine's armour was painted black. Since this time, Abaddon's 'Black Legion' has raided the Imperium, sowing havoc and misery on every world it attacks.
My questions are:
- Is the paint scheme in the diorama incorrect? Has Games Workshop ever acknowledged this?
- If the paint scheme was correct, when was the Sons of Horus scheme retconned into the canon?
Edit. Here is a closeup image: