Men's dress for off-hours

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.''

Sweaters are as integral to a man's wardrobe this season as to a woman's and are also big and roomy. In some cases the styles are almost identical. Ellis's zebra, tiger, and leopard sweaters exist for both sexes. Many of Laura Pearson's Ecuadorean-made hand knits for Tijuca, decorated with zigzags, circles, and other abstract motifs, come in male as well as female versions.

Extra-large argyles, color blocks, and raised surface effects are new for pullovers, loose turtlenecks, and sweater jackets. Many vests come in Fair Isle stripes and, as Mr. Tolbert points out, ''They'll be worn by some men with suits to business.''

Double-breasted looks, which were on the markdown racks for a while, are having a comeback. Slim-hipped European tailoring often means jackets without vents, but the single vent is more prevalent. Shoulders remain soft and just slightly padded, being somewhat broader for international styles. Lapels, according to Mr. Tolbert, are ''31/2 to 4-inch maximum, primarily notched, and for dressy looks, peaked.'' Tie widths have settled down to between 31/4 and 31/ 2-inches.

Colored, striped, or patterned shirts with white or ivory collars are alternatives to pristine white. The fashion cognoscenti add a discreet collar pin or bar. Dashing suspenders are being worn by intrepid stockbrokers, bankers, and upper-echelon executives who, like our British cousins, call them ''braces.''

As they take a more spirited interest in clothes, men are spending more money on them. ''Five years ago a man wouldn't pay $250 for a cashmere sweater,'' says Paula Manikowski of Peter Barton's Closet, a high-fashion sportswear and accessories firm. ''Today - no problem.''

Luxuries are certainly there for the asking. Among this year's: designer Dmitri's midcalf length topcoat of camel's hair with a deep shawl collar of fisher fur. It's priced at $3,000. But the way things are going, that shouldn't stop some man, somewhere, from buying it.