As alpha3six said Istvaan III was as much about wiping out elements of four traitor legions whose loyalty to the Warmaster was suspect as it was putting down a rebel planet.
Often the terran parts of the legions were still loyal to the Emperor more so than the Warmaster even within the Sons of Horus themselves.
So Horus, Angron, Mortarion and Fulgrim all picked units that they had doubts about and sent them in the first wave that attacked the rebels.
Once the battle was well underway the traitor elements of the first invasion force buttoned up and Horus unleashed a virus that wiped out all life on the planet creating huge clouds of methane gas, which he then ignited causing a global firestorm.
Still some elements of the invasion force survived.
Horus was going to continue bombarding the planet but Angron landed with his World Eaters to root out the loyalist marines.
A lengthy campaign ensued in an attempt to eradicate all the survivors of the first wave.
The armour would be cobbled together because the loyalist elements on the planet were obviously not going to be resupplied so as their armour was damaged they replaced it with what was at hand. They obviously were not going to spend the time repainting it even if they had any paint, which they didn't.