He didn't... these are digibashes, not actual repaints; that is he's got a photo of the figure (from the Seibertron toy galleries) and used photo manipulation software to recolour it.
Regarding the light grey bits, I've found the best way to deal with recolouring them is to mess with the different layer types, and sometimes even to overlay a couple to get the effect you're after.
Here's a quick and dirty Bludgeon one I knocked up...
For the green layer, I painted all the areas I wanted green on a Multiply layer, but the light grey still doesn't bring out the shadows much. So I duplicated the green layer, switched to a Linear Burn and reduced the opacity to 30%. So the Multiply layer darkens the colour and provides most of the hue, and the Linear Burn layer deepens the contrast to bring out the shadows.
The yellow areas were just done by painting yellow on a Hue layer.
This was done in Artweaver, but if you mess about in whatever program you're on, you should be able to find a way of combining the right layers. Another thing of course is to select areas where the tone is too bright or the contrast too low and play with the levels.
As for the camo, here's my attempt at eliminating it (only on the chest).
I did it by using a small airbrush and using the dropper to pick up areas of colour near the edges of the camo. Then rather than stroking the brush across, I stippled it. I also continually selected new colours, rather than just picking the one and doing the whole area. This helps it looks less artificial, I think. But with all the airbrushing, I found I lost the front edge of the chest, so I just drew a couple of slightly softened white lines to sharpen it up.
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