Sorry for the belated reply to your question.
Yes, some of the yellow has returned in part. After two years in storage the whitening process is only a temporary solution. Although the yellowing is not like it was before, I would say as a rough estimate that the fort max parts I still have (I ended up selling the one in the photos except for a few parts) about 25 percent of the yellow has returned. And this is after storage in a dark cupboard!