At the same MET Gala, Mary Kate wore a color-drenched, floor sweeping Givenchy, also vintage. This is a completely monochrome look--even the little clutch is red!
Don't be intimidated by head-to-toe color. Most of us are just fine with all over grey or black, for instance, but red? Why not? This kind of quirk is the key to the Olsen's style.