What separates the Missionis from the rest of the international fashion crowd is not only their 25-year business and marriage partnership but their refusal to crow about it.

"We don't think what we make is fashion," says Tai Missoni who, with his wife, Rosita, creates the Italian made Missioni Knitwear collection that has long since made a handsome name for itself in the field. "We make pieces to put on to make you feel more comfortable."

This is how it works. Tai does the fabrics, which are always knit - "It's my profession, it's all I know," he says - and always in natural fibers such as wool, cashmere, mohair, silk.

Rosita then takes the fabrics created by her husband - "I never totally reject anything he has done, but let me say it is a hectic collaboration" - and makes the designs for the clothing.

Together they develop a palette of about two dozen colors, leaving store buyers (and customers) a wide range of patterns, textures and shapes to choose from.

If the Missionis were not the first to mix patterns and textures, their prestige was enough to make the concept catch on. And to further encourage women to mix rather than match, the couple makes several trips annually to this country from their home and factory in Sumirago, near Varese in Nothern Italy, where all of their fabrics and clothes are made.

Last night they drew about 200 guests to a Folger Library benefit fashion show presented by Bloomingdale's, where their designs were combined with costumes from the Folger and from Arena Stage.

(It was a night for such things. Lord & Taylor had designer Bill Blass at a benefit for the Corcoran Gallery of Art across town.)

This year the Missoni styles are generally loose fitting, with very light fabrics that often appear to be handknit, though they're made in the Sumirago plant. "When you walk," says Tai Missoni, "you get the feeling you are swimming in the clothes."

"Most women have no fantasy," says Rosita Missoni about the campaign to have customers mix, not match. But for herself, she sticks mostly to black. "I'm around color so much all day long, it is casier for me to be in black. Besides," she adds, "it suits my geometry."

Mrs. Missoni abandoned her typical black wardrobe for an outfit all in red for her son's wedding recently. Her son, Vittorio, who runs the firm's shipping department, often teases her about her wardrobe being "like the closet of Zorro."

But for Tai Missoni, clothes are a mix of miltipurpose designs that the comfortable and easy to wear. Yesterday, at lunch, he was wearing a black cotton T-shirt (a Missoni) under a mohair shirt (American make) with a mohair plaid snap shirt as a jacket, plus American jeans, white socks and loafers. So what does a man with such imagination wear on black-tie occasions? "The important thing is black shoes," says Tai Missoni. 'Forme that is subito sera (instant evening).