“We wash our clothes and cut our hair on Erev Rosh Hashanah”: to show that we trust in His kindness, that He will bring our judgment out in our favor. Even so, on Rosh Hashanah one should not wear embroidered or silk garments as on other Yamim Tovim, so that the fear of the judgment will be upon him; rather, he should wear fine white garments (Taz, in the name of the Maharshal). And see the Magen Avraham, siman 597, that where it is not the custom to wear white, at least he should not wear such distinguished clothes.