Alex,
You pretty much said exactly what I would have said. There were 6 complete suits and the parts for the suits were never well defined and STILL aren't well defined. Pieces have floated back and forth from suit to suit over the years as well as during development. We often make broad sweeping statements like "The first supertrooper became the PP1 which became the PP3, which became the ESB Hero." While this statement is true in a broad and general sense, it may not be true for every single piece of the suit.
One of the themes we have come back to again and again and again over the years is that because we microanalize the details of the suits we tend to create these extravagent explanations for why certain things are done a certain way. However, the more we have learned the more we have come to find that usually whatever is the simplest, cheapest, and quickest answer is usually the correct answer.
In regard to the whole repainting of the shoulder studs, as Alex pointed out, there had to be at LEAST 6 sets of studs... meaning at minimum 32 studs. From what we have seen different suits were painted at different times, by different people, and a lot of the pieces were later repainted for other scenes/movies. It is unlikely that there was a conscious decision to change the colors of the studs as much as it just happened during the process.
Are the ESB studs white? I don't know, but I could easily buy it. The supertrooper studs were a gloss white, just like the rest of the suit and there is no reason why some of the studs might not have made it into ESB without being repainted. The PP1 appear to be silver or light grey. The PP2 seem to be silver painted over white. The PP3 appears to have one dark grey stud and the rest appear to be either white or silver, with one having the red top. As you can see... they are just all over the place.
As for the ESB studs... my personal opinion is that it changes... I know.. you didn't want to hear that, but in some scenes, I feel like I see the same silver/white mottling that is seen on the PP2, especially when Fett is in Slave 1. Just looks like they are silver and some of it has rubbed off on the edges. In a lot of scenes it is unfair to judge because of the harsh lighting coming from overhead and blowing out the studs, making them appear white. However, there are a few pics (mostly publicity photos) where they truly do appear to be solid white. Very very hard to say and I am not definitively convinced one way or another. I know that isn't what you guys wanted to hear, but I just don't have a solid answer... of course the answer could be that they changed throughout production and they may have simply grabbed up different studs at different times... who knows.