Do you have yellowed brick and wish to restore it to its original color? I recently stumbled upon a Maker Magazine article called "Un-Yellow Old Plastics - Retr0brite!" ((URL) I think this might work for yellowed white and grey brick. According to (...) (16 years ago, 9-Mar-09, to lugnet.general, lugnet.mediawatch)
(...) ... Yes. I tried this last week and the results were very good. Take a look at this (URL) post>. The picture shows the results before and after a exposure of only 2 hours in the sun. We are having a heat wave here in Brazil and I was afraid of (...) (16 years ago, 9-Mar-09, to lugnet.general, lugnet.mediawatch, FTX)