White symbolizes simplicity because it includes all colors. It needs nothing else, because it has everything. One who has found that inner light, that white light, needs nothing. He is in perfect harmony with himself and with the Almighty. The greatest beauty is that which does not need outer enhancements.
True beauty shines forth from within.
The Kohen Gadol would enter the Holy of Holies on Yom Kippur, the holiest day of the year, clothed in simple Bigdei Bad - Bigdei Lavan, white linen vestments. This was the zenith of his service, the most sublime moment of his year - and he was dressed in simple white garments. He represented the entire Jewish people - unified together in teshuvah and tefillah, prayer. All focused together on this one day towards one goal, one G-d. This was his avodas Yom Ha'Kippurim, a goal to which we should all aspire all year.