I got a few questions about how I did those gameboys.
so:
I used Montana paint. From the mtn '94 range.
http://www.mtncolors.com/products/56-94
These are pretty bright coloured paints with a matte finish. Since the gameboy is painted from the inside, it's still smooth. I like these paints because they seem to stick on pretty much everything, but it's whatever paint you like best I guess.
I sprayed maybe 2 layers of colour first, and finished of with a clear coat, to prevent scratching when assembling the gameboy again.
I don't have a lot of under construction pics, sorry. But basically, what I did, was first start with taping the complete outside of the dmg (which is a bitch to do, but take your time, because it's easier then having to clean the outside later on.)
then I lay both halves down, and cover the bottom part with just a piece of paper. spraying basically just the top, and creating a gradient from blue to nothing.
the thing with painting from the inside is, you can't redo something when you went to far. If it was a normal gameboy painted from the outside, you could just paint it all blue, and layer the green on it later..
once you have one colour on, you can paint the bottom. If the first gradient is ok, you can basically just fill the rest.
One tip I can give, is to make the front part of the tape easy to peal. so you can check halfway if everything is covered. (screwholes and parts behind bits of upstanding plastic are hard to reach sometimes.)
Anyway, most of it is just knowing how to handle a spraycan. Long strokes from left to right, always start and stop spraying away from your subject. (to avoid drips)
You could practice with plastic containers for example.
that's about all I can think of right now 
