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.
- 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.
- Apply a thin layer of Primer paint and let it dry. 10 minutes should be enough.
- 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