Have you considered doing him in
NL colours? I've seen some fairly good renditions of Kharn in the blue with those lightning details.
Though if that's not your style I'd say go post heresy colours.
I'd also suggest doing some minor kitbashing in order to closer represent the Betrayer rather than the Bloody.
Give him a chain axe, the familiar khornite helmet, chaosy backpack, and some more skulls.
This is merely a suggestion. If you are content with the Kharn the Bloody model then by all means, paint away
I look forward to seeing the result
Peace