Answer: Bromine

More often than not, when you come across a really old piece of electronics that originally sported a gray or beige case—like early Apple II computers, Commodore computers, the Super Nintendo Entertainment System, and countless other devices like computer monitors—the plastic is now an unsightly yellowish brown color.

While discoloration occurs over time in a wide variety of plastics, especially those exposed to the ultraviolet light of sunlight or fluorescent office lighting, only some of them seem to get that stained-tooth coloration. What gives? Why does one old PC monitor fade to a lighter shade of gray in the sun, but another old PC monitor takes on the coloration of a cup of tea?

The secret, unseen to the naked eye, lies in the chemical composition of the plastic. Millions of products during the 1980s and 1990s were manufactured with light-colored cases made out of ABS (acrylonitrile butadiene styrene). In its pure form, the common plastic is far too flammable to be used safely in consumer products—especially those that house parts, like a malfunctioning power supply, that could literally go up in smoke. To make the plastic safe, the manufacturers used a variety of flame retarding chemicals. One of the most popular chemicals due to its abundance and low price, was bromine.

Although time alone is enough to begin to yellow plastic (so even new-in-the-box computers from the era will be a little yellow), these older bromine-rich plastics react very strongly to ultraviolet light exposure and take on a dingy yellowish-brown tinge (ironically not far off from the brown color of the original liquid bromine mixed into the plastic). Often times, this discoloration results in odd contrasts as the ultraviolet light discolors the different plastics used in the product at different rates. You can see this effect in the photo of the old SNES console here—the plastic used in the main body and the plastic used in the cartridge bay section were different and aged into an odd two-tone pattern.