Hey bud I have the same seat!
I dont a perfect white one exists anymore!
Anyway, Here is a fix or a HELP to fade the yellow process.
Take the seat out of the bag, dont store it in dark places anymore and
are you ready for this! Put the seat in the back window area of your car for a few weeks! Make sure its IN THE SUN to get as much SUN as possible! Its the best bleaching process for white plasctics.
Hope this helps, it did for me.
L