This ones color is alot similar to Korax's.

How about a palegreen with dark brown eyes.

That looked quite good I think, maybe a bit too green. I also made the teeth grayish.

That's also nice, but it simply doesn't fit the "bone-theme" quite well. Try to use a bone'y color, and maybe make the eyes black, just as there are just holes in his head

Squatten the torso, do the black eye thing, and add new protrusions coming from him!

looks like a chaos serpent

Just using an imagining program, sorry if it isn't much help to the actual editing but I figure it gives some ideas. Here's another without eyes:

Korax was originally meant to be much like D'Sparil, a two-stage boss monster first riding a chaos serpent and then on foot. Then it was changed because it deemed too similar to Heretic.

So, originally, what we know now as Korax was going to be the mount. And the design for Korax himself had become the Heresiarch to provide some mid-boss battles.

So, if Korax and edits of him look like a Chaos Serpent, it's because he was originally designed as one.

@Gez - Thanks for cleaning this up

BTW,
That's nice, actually perfect (didn't see the nice color tint at work)

I refer to recolor

An imp or cacodemon with that green color wouldn't have elicited comments about looking like a chaos serpent, so I rest my case.