Painting by the numbers
The default surface
I started by painting with the basic grey, because most of his body (or frame, him being a rohbut?) was going to be that. This was nicely paintable in one go, because there were enough !grey parts to hold him on while painting around.
The dark greys
Megatron's arms, the Fusion Cannon and most of his legs were to be painted dark grey. I took this stage as my next one, as my process seemed to be "the bottom-most layer first", not totally unexpectedly. With the feet themselves I made a bit of mess, and had to fix them later.
Originally, as stated above, I was going to do the black parts with German Grey instead, but the contrast difference between that and the dark grey was so small that after the first parts I redid those with flat black instead. It proved to be the best choice, silly me had thought I knew better. In addition to both gun business ends, Megatron's pants, the scope's lense I also painted his mouth and robotic unibrow. Before these I had done his face with a lighter grey, to give it a proper difference from the rest of the body and the head.
To complement the greyscale, Megatron had a few colourful spots on his abdomen: some blue, red and yellow bits. His eyes were also red and his upper arms had red bands, not to forget the red button on his Fusion Cannon. For some stupid reason I had some doubts about the upper arm reds in this scale, but as they worked fantastically in MP-36, I knew them to work here too.
Technically he was now done, except for the decal on his chest. I just wasn't ready to call him (or myself) done at this point.
More!
No, that wasn't enough. I wanted to improve the details according to my own madnes. As the first extra step I painted the eye-bit of the Fusion Cannon / scope's rear bit's edge to be more rubbery (VMA Tire Black) and redid the lens part with black. Megatron's gun mode's gun barrel's insides got a black layer and a follow-up with drybrushed Gunmetal.
Of course I couldn't leave this just on normal paints, because they looked a bit too flat to me. The greys I washed with the grey wash and the yellow/red/blue bits I coated with the satin varnish to make them stand out a bit differently from the rest of the body. That gloss varnish I only applied to the scope's lens.
Obviously the more or less undisputed leader of the Decepticons had to have a roundel in the middle of their chest. So that's what I did: I applied one violet decal on Megatron's chest. This time I had no complaints about the decal work, for a change.