Use a dusty brown as a base colour and vary different shades of green throughout the army so you've got a unifying colour but they dont look too regimented. Also the different shades of green would be suitable for different terrain types (except snow obviously).
e.g
Dark Angels Green + Chaos Black= Urban
Snot Green= Woods/Jungle
Bright/Yellowish green= Desert
Catachan Green= Ash Wastes etc |