Cleaning "yellowed" pieces

Hello, everyone!

Does anyone have any suggestions on this? A few of my old 12v track rails
and track are yellowed. I also have some white that has done the same thing.
This has always mystified me because out of 16 pieces of rail assembled when
new, only one or two of them will do this. I also had a set a while back
where one 2x3 white brick did this while all of the other pieces remained
pure white. Are there any preventative measure when assembling a new set?
Should all of us wash the parts in detergent before even putting a new set
together to get rid of any residual manufacturing oils (which is why assume
that the bricks might do this)? Any suggestions would really be appreciated!

Best Regards,

Mark