Is there a Torah source for the Kallah Bride to wear only white? What about a second marriage? Does the white indicate that the bride and groom make the day before marriage a special attempt to do Tshuvah?
Nitei Gavriel Nisuin 1 15:1:4 mentions this Minhag in the name of Maharam Mintz, Likutei Maharich and Shulchan Haezer. He mentions that some Chasidim do not wear total white as it is Chukas Hagoyim.
Based on this I would say that there is no source in the Torah that requires such. However the fact that a majority of Klal Yisrael does so should make one hesitate to change the Minhag.
The closest is the gemara at the end of taanis where the girls would wear white on tu bi'av trying to get married. There are also two places in the zohar that mentions the brides dress color. In one place he says white. And in one place he says red. But i would not be able to find the maareh mikomos presently, so don't bother asking.