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Here's what I turn too when I'm stuck on colours:
http://www.pixeljoint.com/p/2203.htmStudy some of his colour choices. It will help.
On the edit I tried to make the colours more interesting and I removed most of the dithering.
Really, dithering can work on metal, but mostly only if you want rusty or dented texture.
Further I do think the colours leaned too much toward stone. It's metal, metal reflects alot so colours from the surrounding can reflect on it and create interesting hues.
And perhaps working on black is not a great idea. It can show you the wrong display of colours. Grey is a nice neutral background colour.