Dear Simple,
I have no problem if you wear a white shirt or a pink shirt. Whatever works. I never said to wear a pink shirt in Yeshiva. It won’t work. The discussion has moved on to now that we do wear white shirts, does it have significance or not.
Historically, there is Slobodka, the Yekkes, and some of the Galicianas. What does this have to do with the Maharil Diskin? I doubt he instituted any change of dress. Though he very possibly said that about it. Which would make it meaningless. And in our day; pointless.
Mesivtas in the US did not require white shirts until the eighties. (Or so I was told. I was also told, that there were more white shirt yeshivos in the sixties then the seventies.)
I hear that Chofetz Chaim switched to white shirts. Because people were assuming that it was not a solid Yeshiva. If it is true, that pretty much destroys your point.
The idea of identifying with Yeshivos based on what you wear, would have been completely absurd to our grandparents. Even though each Yeshiva had it’s own style of dress in those days, anybody not part of the Yeshivah dressing like that, was assumed to just like that particular style. Levush is when you do not have any other connection. The Rebbe does not need the levush. Nor do his dedicated Chassidim. Those that are loosely affiliated benefit the most from a levush. That idea would not make sense in a real yeshivshe velt.