Well assuming you are like me and want some real depth to the model.... be prepared to forgo a little bit of reality and do a colour that is far lighter than your Black Legion and add some grass.
I know you've seen my Black Legion (I appreciate the compliment) but I even found "bright" green just felt a little too crazy so I went with dead grass. The sort of thing you see when water is allowed to collect near a crevace or whatever.
It does help. Healthy sprinkles of cork for brighter rocks helps too. It's very easy to make an 'alien' rock look however you like. I often use 'blues' and 'greens' to varying degrees in shading a rock down from a bright grey.
I've even seen some basing companies doing great stuff with weeds, or wild flower. Very cool stuff you could always add as well.
Finally there are some great companies doing resin bases with slate that really add a lot of dynamic poses to your army as well as colour.