Personally I wouldn't care much about how the model was painted regarding the
WYSIWYG and Daemons of Whatever. The
WYSIWYG factor for me is more the gear and weapons the model has, so if you had maybe just a little Nurgle Icon somewhere to remind me he was a
DP of Nurgle that would suffice, Nurgle can be hateful and angry too.
However, if you are competitive and the tournament(s) in question have a theme score you may suffer a few points for that. If you can pull it off, a sort of Nurgle
DP succumbing to the rage of Khorne to fit in with your other guys, then that would be pretty neat. Frankly I'd go the
DP of Khorne though with a Greater Etherblade, then on the charge he'd be S8 for instakilling
MEQs (at least most of them). Tis true, though, the Nurgles get a nice cover save along the way!
Just Grimoire them and you won't have to worry about the cover though