Not sure why every computer was that beige color, but I can certainly answer the question about discoloration! Plastics tend to discolor and degrade over time due to UV radiation (sunlight) and heat. Many computer cases also contain added flame retardant chemicals which also degrade over time. So basically, what your seeing is analogous to a metal car rusting. The problem happens with most plastics, but the problem is more noticeable on computers (or my old SNES) because of their light color.

As for the reason computers are beige, I can only speculate that they did it that way because it is the cheapest and easiest. Computer cases are made from ABS plastic, which is naturally that off white. Making them a different color would increase costs. I would speculate that one of the reasons most plastic computers now days are black is because it shows that discoloration the least.

edit: left out a word

I can answer that!

The color is actually a light grey which yellows over time.

I worked at Siemens in the early 90s, in the Usability Division. That was a business unit which would test and design user interfaces, usability of devices, the external appearance of appliances etc.

My boss back then was a member of the Deutsches Institut fuer Normung so he knew all about specifications.

I asked him the very same question because all the Siemens-Nixdorf PCs had exactly the same, light greyish color and I was wondering why they didn't make them more interesting (they did produce horrible, lime green laptops for a while back then.

Apparently, this grey color was specifically designed to appear neutral to the human eye so it would not distract the user and blend well into an office environment.