Ooh, there's lovely. That's a nice mini, there. Yeah - he's non-legion specific, but as you say, he lacks for that pustulent dripiness, lascivious fleshiness, and uncontrollable mutation that exemplify nurgle, slaanesh and tzeentch respectively. So, Khorne it is.
Blood for da blood god, skulls for da frone of skulls
(or, as the mantra goes over here "paint for the paint god, bitz for the box of bitz...")
or not, as the case may be...
Before I start on colours, some fluff will help explain details - I'm gonna try for a potted history of the chaos legions in one small paragraph. Ok, here I go...
10,000 years before the 40k "now" the imperium was a great big galactic empire of niceness and somewhat over-zealous expansion. Then Horus (the emperor of humanity's favoured son) decided worshipping chaos gods was a good idea, and about half the space marine legions agreed with him. There was a gigantamongous civil war in which lots of space marines killed each other. The chaos worshippers almost won, but at the last minute horus screwed up and the emp kicked his ass personally. Horus died, the chaos legions all ran away to daemon-lands (the warp), and the emp ended up a withered dead dude on life support for 10k years. This is why regular and chaos space marines dont like each other. In the warp, time is odd, so lots of the original traitor marines are still there, plotting revenge. The 10k years still passed, though, so they've all gone a bit mad and gribbly and mutated from living around so much magic/psychic power/daemons/general oddness whilst nursing 10,000 years of grudge.
OK, done.
The chaos legion that turned to Khorne was the World Eaters. Their pre-heresy colours were white with blue trim. The shoulder icon (and banner icon) was a set of stylised red jaws enclosing a world (sometimes bleeding, sometimes not). They were always a bit berserk, but completely lost the plot when they turned to chaos. Post heresy, they painted all their armour with blood, but retained their iconography. They also use a lot of brass (not gold) and skulls.
much like this, in fact:
http://warhammer40k.wikia.com/wiki/World_Eaters
So, as for the mini, my recommendation would be as above, basically. The exact shade of red is open to interpretation, so feel free to darken it down a bit.
You could incorporate one white section with blue trim to show old allegiences. This would show off any drizzled blood stains nicely, I reckon.
As for contrasting colours areas, it's entirely up to you (cloak, etc) . There are no colours that would be un-fluffy.
I notice he's holding a space marine helmet. Khorne has a known rivalry with Slaanesh, so a pink helm would be OK (but would stick out, visually). The World Eaters have a rivalry with pretty much everybody, but Space Wolves would be a good foil, as theyre quite bonkers, too.
The gun he's carrying is a combi-melta. The lower barrel is a bolter (standard sidearm) and the upper, big barrel fires microwaves. Many people paint their melta gun barrels black from the heat, but as I understand it, technically it wouldnt get hot at all (unlike the target, which inevitably explodes.)
Hope that helps. Any other questions, please ask
