In short, the type of plastic that Commodore used to make many Amiga computers is the reason it becomes that gross yellow color over time. A type of plastic called ABS, or acrylonitrile butadiene styrene, was used to make the Amiga and other electronics of the time. Because ABS is combustible, manufacturers mix a variety of flame retardant chemicals into the mix to help reduce the chances of catching fire. Bromine is the chemical most often used and it has a naturally brown color. The problem starts when bromine is exposed to ultraviolet radiation, which is found in natural and artificial sources of light. The reaction breaks bonds that were created in the manufacturing process which causes oxidation. The oxidation of these free bromine particles is what results in that sickly yellow color appearing on your computer. That certainly doesn’t explain why pieces age and color differently on the same computer. One hypothesis is that Commodore didn’t get the mix of ABS and flame retardant right, resulting in the yellowing color you see on many Amiga computers today.