Is it strange that my little brothers dreamcast has yellowed in streaks on top? I havent heard of dreamcasts doing it before? Also my Tenokoe 2 has yellowed to a browny colour and my flap door thing on my nes is starting to yellow.
Good thing most sega consoles are black 
Any grey/white/platinum plastic has the potential to yellow over time. It has to do with a chemical breakdown at the molecular level. I'm sure the same breakdown happens with black plastics, you just can't see it.
There are certain factors that determine if/when it will happen, namely the type and quantity of flame-retardants used in the manufacturing process.
I've also heard that certain types of light, specifically sunlight and fluorescent, cause a reaction that jump starts and expedites this breakdown.
Also, the reason you get different pieces of say, a console yellowing at different rates and times is because the different pieces are manufactured in different places by different processes. Some of these processes add more of the necessary chemicals to the plastic than others.
