Since he launched his label three years ago, it's seemed as if Jason Wu has been auditioning to replace Oscar de la Renta or Carolina Herrera. He certainly has the immaculate Park Avenue thing down cold, should either of the legends decide to step down. But for Spring, inspired by a trip to Japan, where his business has been growing like gangbusters, Wu added an element of quirky whimsy that should broaden his appeal beyond the young ladies-who-lunch set. It came through in the form of a fuchsia cashmere T-shirt worn with an embroidered, dotted micro-pleat skirt, and again in the mix-and-match neons of a cardigan, a bow-front blouse, and a full A-line skirt. Not that the designer skimped on the ladylike numbers upon which he's built his socialite client baseFrench knot embroideries and Swarovski crystals decorated a white silk faille coat and a creamy organza dress, both of which erred on the prim side of polished. For the money, a fits-like-a-glove black-on-white floral tapestry sheath was a standout. The evening gowns in jewel-tone chiffon that closed the show successfully walked the line between Wu's two worlds. You could imagine an Olivia Chantecaille, who sat in his front row, wearing the emerald green dress with diamonds and heels, but the right downtown girl could also pull it off with flat sandals, a stack of gold bangles on both wrists, and a tan.