Sorry if this post is a bit long-winded. I started with the classic grey shell, but it had a beige tint to it which does not look much like SNES grey, so I decided to paint it. I have an airbrush and mixed my own colors for the light and dark grey. Here is a pic after I painted and drilled out my holes and cut out the screen. My screen work could have been cleaner, but the exacto was not easy to control. It will all be covered by the screen surround though.
As for the button labels and logo, I knew I did not want to go with waterslide decals. I really wanted to figure out how to make it seem like it was factory printed, so I decided to get some vinyl stencils made. I knew I would not be able to fit the button letters on the right and left like the SNES picture below. I also knew the circle would be too large for the Game Boy.
I do not have Photoshop, so I did the best I could with Pages on my mac to rearrange the button letters to be on top and bottom. The Gameboy button letters are on the bottom anyway, so I thought it would mesh both styles. This is what I came up with.
The image looks rough, but when I uploaded the file to the online decal site it took out the weird imperfections. I also made a smaller circle and it let me install custom fonts. So this is what I came up with for my stencils.
I was worried that these decal would pull off my paint. In fact I tested a small section, and it did! So I had to airbrush that spot again to fix it. I decided to put a clear matte spray coat over it. I was going to do that at the end anyway to protect the paint job, but I thought maybe the decals would come off cleaner that way. I got the decals in place and repositioned the button letters individually because they seemed a bit odd. I originally wanted the circle to be entirely on the front face, but when I tried that it was not centered over the buttons and that drove me crazy. So I ended up letting it over-hang on the top and right side just a bit, but I thought that just might work out ok.
I covered up the rest of the shell that I did not want painted with dark grey and airbrushed over the stencils. This was the scary part. I was concerned that the decals might not have made a good seal and might let paint bleed through. I was also concerned it might still peel off the paint. Even if both worked out ok, then I thought, "How do I get all the small individuals decals off with out scratching it up?" I took my time and removed the individual decals very carefully with a small pick. I was very pleased with the results.
Now this is where I give another shout out to VeteranGamer and daftmike for the awesome idea to do a SNES cart, except I am going with the US version. I had mine printed in grey resin so that it would have a smooth surface.