More results:
The Good
My Famicom
A friend's Famicom
The Bad
Western Electric Telephone
Use the handset color as a reference point, I left it untouched.
I think this is what they call white blooming, where you get patches of excessive discoloration:
I blame the Oxy-based product I'm using, it's Vanish Crystal White and the ingredients list says it contains an 'optical whitener'. I believe this compound may have helped the PC case and the Famicoms since they're white but it hurt the telephone.
Super Famicom
Now this makes me a very sad panda since this is my only Super Famicom 😢, looks like a combination of white bloom and not enough treatment:
Again I'm blaming the whitener in the Oxy but I'm not really sure. I think I'll try another go with regular Vanish and hope for some improvement. 🙁