IT HAS BEEN DIPPED IN ACID, PUMMELED WITH STONES AND BLASTED WITH SHOTGUNS, BUT THIS IS ONE FASHION TREND THAT REFUSES TO DIE. THAT FAVORITE OLD AMERICAN CLASSIC -- DENIM -- NEVER SEEMS TO COMPLETELY DISAPPEAR FROM STYLE; IT JUST TAKES ON A NEW LIFE WITH EACH FASHION CYCLE. LAST YEAR'S RIPPED AND FADED DENIMS HAVE BEEN ECLIPSED THIS SPRING BY COLORFUL RED, GREEN, YELLOW AND BLUE JACKETS AND JEANS INSPIRED BY THE BRIGHTS OF THE '60S.