Just about the time your ranunculuses start to bloom, a riot of spring garden colors will sprout in local stores. Evening dresses, especially, have petal hues that range from tea-rose tones to sizzling anemone shades. Many gain added shimmer from the shiny fabrics or glittering sequins with which they're made--designers' ways of pepping up a season in which basic black, navy blue, beige and gray are the keynote colors for day and night. Here, a sampling of the best and the brightest for spring, photographed on runways in Paris and New York. At left, Bill Blass' rosy skirt of plaid taffeta with pink sweater set to match. Below, Emmanuel Ungaro's tulip-toned, satin-jacketed evening suits. At right, Geoffrey Beene's homage to the flower garden he'd love to have.