Nice work. The green and tan/brown always look good together in my opinion.
For sci-fi minis, I often paint the guns black and drybrush with a steel color. I think it gives them some character.
I did this with some Exodus Wars 6mm minis. I started with a similar airbrushed technique, though I chose green and dark gray as the camouflage colors.
A few photos are here:
http://texaswargamer.com/6mm-sci-fi/6mm-exodus-wars
Also, here's the same technique used with a Mark V Ogre:
http://texaswargamer.com/6mm-sci-fi/6mm-ogregev
The Ogre is at the bottom of the page. Note - I copied the camouflage scheme from someone else.