I had an extra Vintage ROTS Clone Trooper laying around (it was the first release with the missing paint aps) so I figured now would be a good time to experiment with the methods outlined in this thread. I also picked up an extra 212th Clone at WM this morning, so I could use the eraser method on one and the hot water method on the other. My apologies that I don't have a camera for documentation purposes, but I'm hoping I can verbally describe the quality of the improvement/outcome in a satisfactory manner.
ERASER VS 212th CLONE TROOPER: I went this route first, as the 212th helmet was much greener than the white ROTS clone, and I figured if the eraser didn't work, I wouldn't cause a significant amount of damage to the helmet. I used (simply enough) the eraser on the end of a Number 2 Paper Mate pencil, since it was small enough to get into the cracks of the helmet itself, and I could grasp it easier than I could the old block-style eraser. I was a little disappointed with the results, though, as there was only a small amount of discoloration on the eraser once I gave the helmet its first pass and the helmet was still pretty discolored. This may have had something to do with the eraser itself, though, so your mileage may vary. That said, I was disheartened enough with this method to jump straight into the semi-boiling water method and abandon the eraser method completely.
RESULT: Yellow Helmet beats Eraser, unfortunately.
BOILING WATER VS 212th CLONE TROOPER: So, since I'd already started on (what I assumed would be) my quest to destroy the 212th Clone Trooper helmet, I went ahead and tried the warm water trick. I put about 1/3 cup of water in a microwaveable Campbell's soup cup, microwaved it for 1 minute at full power, and then dropped the helmet into the water once it was out of the microwave. I then used a glove on my left hand to hold the helmet while I extracted the helmet from the water using tweezers, dipped a dry cloth into the cup, and then rubbed the 212th helmet briskly. This worked far better for me than the first method, as the vast majority of the discoloration came off (or at the very least lessened considerably), and the helmet was notably whiter.
Being a pessimist, though, I wanted to try it on the plain white Clone helmet (since I felt the orange markings may have been playing tricks on my rods and cones and simply making me THINK it was still a yellowish tint when it wasn't). I followed the same procedures that I did with the 212th helmet and the boiling water, and this time it was REALLY noticeable. Then I proceeded to drop the vintage Stormtrooper helmet, my "correctly painted" ROTS Clone helmet, and my first 212th Vintage Clone helmets into the water, with similar results. Again, though, being a pessimist, I can still detect some slight discoloration, but all of the helmets I tried are now considerably whiter than they were before they hit the boiling water.
RESULT: Hot water beats yellow helmet.
I'm glad that the ones I have already are now "fixed", but this should definitely NOT be something I have to do with every Clone from here on out. Hasbro needs to address this and correct things now, so we don't have to take the time and fix their errors on our own time.