Sensible old sneakers, which have been doing all kinds of drudgery on American feet for more than 70 years, have suddenly, like Cinderella, burst forth in beauty. Once recognized on every dusty road or sandy beach by such distinctive treads as those at right, they appear now, brightly colored, in such unusual incarnations as high-heeled spectator, sneakers for women and hand-sewn Italian-made sneakers for men. There are also such useful varieties as sneaker boots with warm fleece linings and saddle sneakers for the college set. And, still adapting themselves to different sports, new varieties range from Little League Baseball (with rubber cleats) to sailing (nonslip squeegee soles). Pictured at left are new members of the fast-growing sneaker family: high heels with a gingham check ($13, Valentine) and polka-dotted saddle ($17, Domani); yellow flats of hemp with leather sole ($8, Friendly). The woman's red boot is fleece-lined ($6, U.S. Rubber); the saddle sneaker was designed by Vincent Richards for tennis ($8, Dunlop). The corduroy chukka boot has a sneaker sole ($7, U.S. Rubber). For the kids there are red sneakers ($3, U.S. Rubber), for men and women colored hemp sneakers with crepe soles, imported from Italy ($17, Domani).