>>14738781I'm a fan of Vallejo's range, but you could do worse than mixing about 3:2 Codex Grey and Catachan Green. For highlights and shades, mix greys and browns rather than black and white to give more natural tones, and for the love of god keep your paints nice and thin and go by gradual layers.
The other thing is to pick a unified palette (the green in this picture) and a few contrasting colours (gold on the officer, orangey-bronze on the shotgunner) to bring out some details. I've seen technically great paintjobs that look weak because of colour selection.