What with the poufy frocks and pageant hair, many young actresses at black-tie to-dos often seem like they raided their mother’s closets (rather than their stylist’s). Scarlett Johansson, on the other hand, has always seemed self-assured enough to pull off any ensemble. And she’s developed a signature trick to keep her look modern: cocktail-length dresses with shoes in brightly colored hues. At a premiere for “The Island” last summer, she paired red peep-toe pumps with a silvery Marchesa dress. When she recently began making the rounds for “Match Point,” the 21-year-old opted for turquoise Jimmy Choos with her golden Oscar de la Renta frock. The end result? Much more a whimsical romp through the Crayola box than Mummy’s stuffy armoire.