PLAIDS PLEATS & POLOS BY RONNA F. HALL c asual elegance is what you'll see this fall in men's fashions. The relaxed, comfortable look is elegantly enhanced by rich jewel tones of purple, green and amber. Tweeds and plaids are infused with bright color. Luxurious cashmere, silk, suede and leather punctuate the look. Seen in all facets of men's clothing again this season is the strong influence of Italian designers like Armani, Valentino and Versati. "This year's styles are an embellishment of last year's," says Alan Orman of Freeds in Windsor. "The influence of the Italians is more into shape than coloration." Double-breasted suits will be the biggest sellers and Harry Kosins of Kosins Clothes and Kosins Big and Tall in Lathrup Village, says they constitute 50 percent of his fall purchases. David Rogovin, manager of L'Uomo Vogue in Southfield, agrees that the double-breasted 104 Jewish News suit is the most fashionable, and the more relaxed, no-vent style can be worn successfully by most men. "Natural blends are the way to go," says Rogovin. "Our customers look for quality and they can find it in cashmere, silk, and wool-blend suits." Charcoal greys and navy blue are dominant colors in suits this fall. "Navys and greys are the power colors in business suits," says Rogovin. "You won't find any browns in any of our suits." Kosins, however, sees a strong revision to brown coming from the Italians. And Bob Benkert, owner of the Claymore Shop and The Polo Store in Birmingham, says that browns and greys are men's favorite colors. Patterns are subdued and pinstripes are much thinner. Muted glen-plaids, windowpanes, and miniature grey-toned checks are enhanced with threads of teal, mauve, and brick. "This slight introduction of color allow the customer to become more aggressive in Mondi of America mixes patterns, in a herringbone tweed jacket, V-neck jacquard sweater and wool jacquard tweed slacks.