Judging by their wardrobes, a lot of men are leading double lives. They play it safe at business by wearing the approved uniform: conservative suit, white shirt, unobtrusive tie, well cared-for shoes, and so on.
Once away from the office, it's another story. Self-expression takes over. Wayward instincts are unleashed. Men may venture into plum-colored blazers, vividly patterned sweaters, and such sybaritic evening attire as pure silk shirts.
This urge to break out of the workaday mold is pretty widespread, according to men's fashion experts. ''Off-hours, men now have an opportunity to dress in a less serious vein. When they come home, they change personality,'' says Robert Beauchamp, fashion director of Gentleman's Quarterly.
Two factors contribute to the American male's freer outlook, says Chip Tolbert, fashion director of the Men's Fashion Association. One is the growing strength of the sportswear industry (trade reports put its gross increase since 1975 at well over 100 percent), the other the success of designer clothing.
''Sportswear has given a man the option to dress differently from his business attire,'' says Mr. Tolbert. ''Designers are having a permanent effect in all areas of fashion by demonstrating that less rigid styling is acceptable - in fact, needed - in today's world.''
Contrasts in texture, unusual detailings, and more generous proportions are among the alterations introduced by designers. There are, for instance, Jhane Barnes's use of specially woven fabrics on both the ''right'' and the ''wrong'' sides; Andrew Fezza's suede edgings on dinner clothes as well as casual blouson jackets; and Perry Ellis's lengthy oversized topcoats.
But Mr. Tolbert cites a new splurge of color as the main overall change in men's wear. ''Color is the name of the game this fall, in business suitings as well as in tweeds and knits,'' he says. ''There's a lot of gray around; but the worsted or flannel that at first glance seems to be solid gray will on closer examination have an undercurrent of wine or olive. Patterns are subtle, but some tweeds look like every color has been mixed in.''