The painting looks quite good, but the background is very distracting. Try a piece of paper instead. I also agree that painting the base makes a world of difference and gives a better sense of scale. :)
I like the overall colour scheme. The metals might look a little 'deeper' if you gave them a wash of thinned black ink, then picked out the high spots with Mithril Silver or similar. Good rust effect on the exhausts.
Other minor niggle: Real rocks look fake on miniatures (I know, it makes little sense, but it throws off the sense of scale) so you might want to consider painting the base.
Geberally, very nice solid work. I wanted to give a 6.5 but I can't, so here's a 7.
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