Paint it

Most white 3D printing filaments are not completely opaque, and I'm not convinced the original Commodore PET was completely white. Also, simulating the yellowing of the plastic over time is a nice detail.

I didn't need to paint the black parts because they look black and opaque enough, but If you'd like to make your PET even more realistic, I recommend you to paint the white parts with spray paint.

1. Sand the surfaces a little bit with a fine grain sand paper. Not only to smooth the 3D printed surface, but also to allow the paint to stick better.
2. Apply a thin layer of Primer paint and let it dry. 10 minutes should be enough.
3. Apply two to four thin color layers, let each layer dry for some minutes between applications. The color I found that better matches the "Computer from the 70's look" (including a little bit of yellowing) is Light Ivory with matte finish