Id say no more than 3 for red and maybe 2 coats using blue. Its got to be a paint meant for metal if you want it to be see through. Also keep in mind the darker the color the fewer coats you want to use.
Side note: A lot of people think krylon xmetals is the only brand that lets you spray and keep a clear look or if youre doing a grey dmg itll let the lettering sahow through. This is not the case however, there are other brands that still allow for a clear look you just have to find them. Krylon Xmetals is just more available I think. But any spray paint meant for Metal will work.
If you arent worried about see through, id suggest any paint meant for plastics, especially the ones that bond to plastic. In all honesty though if you tape the case off right and paint on the inside it wont really matter what type of paint you use. Because your hands will never touch it. Ive been thinking about doing a black one myself sometime soon.
As for colors that dont really show the purple tint..any bright neon color or deep dark color will probably do the job.
Also a few tips when you paint:
-Dont leave the battery door attached to the back. Alot of people make that mistake.
-Use silly putty to plug up the speaker holes from the out side of the case. Helps prevent any un wanted paint getting on the front of te gameboy. Try as you might Tape will not suffice for this job. If you dont have silly putty use rounded tooth picks broken off and jam them in there. But honestly go buy some silly putty it 1$ at most stores and works alot better with less work and its reusable.
-Get yourself some thin painters tape..the blue stuff. Its awesome and peels off nice and smooth. Apply it firmly around all corners and holes that may allow paint to seep through. If you arent sure all the tape will do its job 100% just stuff some silly putty in the cracks.
Aaaand thats all I can think of. Good luck man.
If it work out and you get to the point of putting custom button in and you dont like the AB being on your buttons just message me and ill send you some pics on how you can cut the case so uou can put gameboy pocket buttons in it like i did with my Blood Boy. Makes it look nice in my personal opinion and more custom. Very simple to do.