If you want a snowy base, you should avoid white or close colors.
I'd go for Behemoth. That's the hive fleet I chose for my own version of Cassius, and I think it's appropriate for yours. Blue and red will pop on the snow, and fit well with your color scheme.
Unless, as suggested by Snrub, you have a tyranid opponent.
Edit : in fact, paint the dead tyranid like the book on Cassius belt.