1) Vents are not needed for heat dissipation and would allow dust into the case.I was thinking lines, like the Apple II series (vents), DMG Game Boy, NES, C64, and probably much more. I think I might be ok with black plastic even for the grey case, to avoid the cost of producing 2 colors (or to include the retro look we can go for aged beige or grey ). I don't think using that shade of grey on the black case would look good. Maybe faux wood paneling.
If it was transparent aluminum it would make this all easier, and we could use clear plastic. You could also make it any color you want. "Aged beige or grey" is fun to say.
Edit: The lines hint at retro consoles and computers, and give extra texture like Tenka and others may want.
2) Transparent aluminum does not exist. Do you mean natural/uncolored aluminum?
3) Spray painting a case would look terrible and the paint would chip off over time.