How about some ultramarines that got tricked into a convoluted scheme by a chaos sorcerer and now carry the blue banner of Tzeentch. Kromlech has some really nice Egyptian themed bits you could use. Maybe have a librarius conclave on bikes with force swords that rides around summoning untold horrors and then charges into close combat? Tzeentch bikers! Sounds great.
Use dark blue but add in gold trim anywhere you can so that the gold is what catches your eye. Use a lot of silver as a neutral colour on the interlocking joints and a blue glow effect on the eyes. Maybe have bolters that are black but edge highlighted with gold? The idea is to have gold be the bright eye catchy colour but with a solid darkish blue on the majority of the model. Then maybe have the pink mutations be REALLY pink... like neon so that it is clear to anyone looking at them that they are in fact tainted by chaos.
http://bitsofwar.com/29-chaos Great stuff to work with here.