It's always a good sensation when the white come back
But be careful guys, the retrobrighting can ruin your plastic over time, and it's now proven that it's only a temporary benefit.
I collect retrostuff since 1999, and made a mass retrobrighting around 2006. The yellowing is here again and worst than before. It seems that peroxide make the plastic more microporous...
Someone tried to retrobright using only sun rays, no peroxide, at it seems to work, but it takes a lot of hours. The peroxide is used to speed up the process, but is not essential.