Every time I see videos or photographs of Haredim of sixty-eighty years ago, I notice an abundance of grey suits and hats, with the occasional brown or grey waistcoat. Examples of this include the famous photo of the Mir Yeshiva in Shanghai, and this video of Ponevezh in Bnei Brak. Approximately when did people start to exclusively favour black, and where did that originate? Was it in Israel? Or the US?
I noticed in artscroll books such as Reb Elyah that the big Rabbis wore black and the yeshiva students did not. I think the movement towards black, etc. was to look like the big Rabbis.
I also see a trend here in Israel, that some older yeshiva students (especially in Bnei Brak area) are starting to wear long frock coats like Roshei Yeshiva, and people tend to address each other as "Rabbi", even though the person is not a Rabbi. (some kind of kavod hatorah philosophy)
could be there's other reasons too. such as to delineate which "Camp" you belong to.