quark2: Regarding ksus yom, I would say that it does have a function. The tallis katan is a garment that used to be worn on it’s own as a tunic of varying lengths. You can still see non-Jews at renfaires wearing talesim ketanim with a belt to hold the sides shut and the neck leaced up. Just because we happen to wear a second shirt over ours doesn’t make the first shirt patur.
yitayningwut: Just a point about the source, it isn’t brought down from the Ari to wear white on shabbos. It was brought down by one of R’ Chaim Vital’s students that he (R’ Chaim) visited the student in a dream dressed all in black. The student asked why and was told that it was his punishment for not wearing white on shabbos.