“why don’t rabbanim (note the term rabbanim, not rabbis) wear colored shirts?”
They are the ones who should wear the white shirts during the week so that we respect them and their roles.
But a Talmid should not dress as a Rebbe.
In the time of the Beit HaMikdash the white Bigdai Kehuna showed the special role of those who were most directly involved in the Divine service.
See the Gemara in Shabbos 119B. Different factors that led to the Desstruction of the first Temple are brought down: Rabbi Yitzchak says because they equated the great and small and Isiah 24:2, “And the nation will be like the Kohain…”. Later a reason is brought by Rabbi Yehudah because they disgraced Talmidei Chachamim (Torah scholars). If boys just past bar mitzvah dress like established Talmidei Chachamim, then it is very easy to disrespect rabbanim.
During the week for the rest of us when there is a a need to dress bekavodig, a solid light blue shirt will do.