One thing I've noticed is that yellowing often does not appear consistently amongst different appliances of the same brand. My old Zanussi dishwasher turned an off-white colour, yet the Zanussi dryer and fridge still looked pristine up until the day they were carted off.
Currently have a Zanussi washing machine, and the plastic parts are getting a slight creamy tinge as time goes by. Most of my other appliances - dishwasher, dryer, fridge-freezer and microwave - are Bosch, and none of these have discoloured in the slightest. Can only assume this is down to varying grades of plastic used in different brands/models. Random indeed!
In the past I've tried removing parts and scrubbing them, running them through a dishwasher, soaking in a hot oxygen bleach solution and even using chlorine bleach. Never made the slightest bit of difference, because the discolouration isn't on the surface, rather it's within the plastic itself. In the end I decided not to let it bother me, because it doesn't affect the function of the appliance in any way.
