I never heard of this whole business, but the only reason I could think of to *not* wear a white shirt is *IF* by wearing a white shirt it makes you think you’re an automatic Gadol HaDor. So if by not wearing a white shirt you maintain a balance and sense of perspective of who you are and where you’re holding, then it makes sense to not wear a white shirt (until it’s appropriate for you). Same goes for a tie and suit.
Imagine 4th graders coming into Yeshiva with suits on every day; it just doesn’t befit them, as nice as it may look.
However, if the standard dress is now to wear a white shirt, NOT because of the above, but simply because that’s the normal decent dress mode even for teens, then it makes sense that they would require white shirts even if they had not previously done so. BTW, this could also be extended to ties and even suits, if that’s what the (non-“faker”/”macher”) norm happens to be.
Just speculation…