A can or two of KRYLON FUSION gloss for plastic spraypaint (make sure it's the exact kind if you want it to look good, as it does not require any pre-paint prepping or sanding.) They are $5 each at ACE hardware, you can also get it at wal-mart or AutoZone, but it's more expensive.
A small flathead or Tri-Wing screwdriver.
A small phillips head (optional, but VERY helpful, unless you enjoy a whole lot of masking tape)
You can get all of the materials at pretty much any hardware store.
Step 1: The Maskening.
Apply masking tape like so. And then open up the battery cover and mask all the battery contacts on the right side of the Gameboy.
Make sure the tape is applied generously, you don't want any paint seeping through or getting through the small cracks and openings somehow, don't rush. This may take a bit, so turn up some dubstep, or your preferred muzacks.
I rushed, so there is still a bit of green on the side of my screen.
Step 2: Deassembelly.
Time to open 'er up.
First, use the Tri-wing, or small flathead to open up the Gameboy, it isn't that hard.
then open the GB, the infrared cover thing will come off, as will the power button, DON'T LOOSE THEM, or the screws.
After that look at the third pic, pull the small pegs on each side of the ribbon out, then nudge the ribbon out, then take out the innerds.
Step 3: The Paintening.
Now that the GB is a hollow shell of it's former self, put it down in a WELL VENTILATED area, preferably outside, it sucked for me because it was waaay too hot out, and there was wind blowing the newspaper everywhere.
Then shake the can, and hold the can about 4 feet away from the GB, then spray it with a MIST of paint, NOT up close, or else it will come out like it did in the second pic, and the paint will come off very easy, and look terrible. After you spray it with a mist, let it sit for 20 mins, and then move it inside, in a cooler area for about an hour, until it's dry.
And if you want a different colored D-pad and A and B button, now would be a good time to paint those.
Step 4: The End Result.
So, after you paint it, and everything is dry, reassemble it.
This was my end result.
I recommend letting it sit for a while before you touch it a lot and move it around.
Oh, and if you Gameboy isn't turning on, it's probably because there is paint on the battery contacts, just scrape it off.