This question is for @Grover
I have been brightening Legos for years now with Hydrogen Peroxide with good success. I submerge the Lego pieces in a glass container with a glass sheet covering to keep the Legos under the solution top. I started with 3 to 4 hours at first but over the years have found that a full two 8 hour days gets the best results. I recently ran into a problem, though. Using the same concentration (3%) hydrogen peroxide solution, same container, and same time length in the sun (2 days) I ended up with over "bleached" bricks. I was working on an old gray (pre 2004) batch. They are now mottled gray with very light gray patches and some are almost white. Even weirder is that a small handful stayed the original old gray. Since the Lego is the same color all the way through I did not think this was possible. I have heard/seen where an edge of a Lego brick that is not properly submerged will have a whiter demarcation for the bit above the liquid (I forget the technical term for this) but this happened to most of the pieces even those totally submerged. Obviously, there are some Lego overlap patterns but I don't think that caused the bleaching. I try not to crowd them but if they have overlapped/touched in the past the worst case was a spot where the yellowing was not removed and the Lego piece had to be redone.
As a chemist, can you think of a reason this happened? Or, any way to reverse this. I hate to throw away 1 lb of old gray parts as they are getting harder to come by.