If you want to use tape, use a good quality masking tape, like blue painter's tape or Frogtape.

I am far from a professional but masking and painter's tape gives me  decent results.  I can't really speak on stencils much as I have always made them using file folders and just held or taped them then sprayed, but that was for putting words/letters/numbers/names on stuff.

If you want to cut stencils, you can try vinyl or other thin plastics, and you can attach it by coating the back of the stencil with a spray-on adhesive for the purpose, and then stick that to the gameboy shell. I have done this.

Removing paint is a pain, but if you try to remove it before it's dry you will only increase your pain, in my experience. Harsh solvents will eat the plastic as readily as the paint, so be prepared for lots of sanding and cussing.

I'm experimenting with acrylic spray paining and stencils (no on the actual GB shells though). And I came across a number of problems. And I was hoping some of you, guys, could perhaps help me out with a bit of advice.

1. Multicolor painting
I tried to use scotch tape to cover parts of the plastic to make multicolor paint job. After I let it dry for an hour I removed the tape and noticed that there is sort of a skirting along the colors boundary. It's not noticeable on camera and from distance but at a closer look it looks like crap. Do you guys know how to avoid that?

2. Cutting stencils
I find cutting stencils very hard but doable. However how do I fix them on GB shell? Sticker craft paper (if yes, is it a good material for stencils)? Magnets? I mean if you just put it on top it can be blown away with paint being sprayed, so you need something to fix it in place...

3. Is there a way to clean out bad paint job?

Thanks!