I agree with Nameless. Blue on the tzeentch model is classic, but it would visually tie them with the deamonettes. The red on the slaaneshi would look good too, but I would go for a pinkish red or a red leaning towards purple. Just plain red would be too Khorny
Thanks for the ideas guys. I'm no closer to deciding what colours to use, but it looks like I might have a bit of time yet, and you've given me something to think about