Wellsir, I think he looks just fine so far (although the armor looks like a departure from his brother-in-arms).
I think for Nurgle one of the best looks is to have the skin in fairly light colors (on anything, daemons/warriors/beastmen/steeds whatever), so that you get a contrast bettween the dark, brooding armor and weapons and the pale, sickly characters themselves. I think that a pallete of pale fleshtones, pale/light greens, and a bit of grey would work well. Look at the Warhammer Nurgle Lord for an idea of what I mean.
If that's not your cup of tea, then I'd say you can never go wrong with bick black horses for the forces of chaos, no matter who your guys worship.