I paint my Sons of Horus Space Marines using an icy blue/green color for the armor, contrasted with orange details. The base coat for the blue is Citadel’s Baharroth Blue, washed with Nuln Oil, and drybrushed with Skink Blue and Hellion Green.
Paints used (with Amazon affiliate links):
Blue Armor:
Baharroth Blue
Nuln Oil
Skink Blue
Hellion Green (seemingly no longer being produced!)
Orange Stuff:
Jokaero Orange
Fire Dragon Bright
Metallics:
Leadbelcher
White:
Ulthuan Grey
Base:
Screaming Skull
Vallejo European Dust
Pallid Wych Flesh