Fresh off my victory with the new Chaos Space Marines army, I put a little time in last night on the Helbrute. You can't give them specific marks of chaos, but I decided to paint mine up Slaaneshi. I will probably convert another one to be Nurgle at some point.
So here it is at present, press fit for the picture. The only conversions currently are the power fist and the head, which I wanted to have a helmet rather than the crazy face that comes with it.
I'm just going for a non-specific industrial ruin look. I'm doing the whole army like this.
The power fist on the Helbrute is a little stubby for my taste, so I modded it to have longer claw fingers. You can see the severed fingers on the left. The new fingers are the horns from one of the CSM heads on the accessory/champion sprue. It actually the same head that I used for the Helbrute's head-swap. I really like the longer claw look.