DIANA VANCE LOOKS tall, slim and elegent in her periwinkle suit, a white fabric flower pinned to her shoulder, her rust-colored hair perfectly coiffed; her nails, painted a cinnamon color, are long and manicured.

Looks aren't the only thing striking about Diana Vance. With perfect aplomb, as though people say such things in everyday conversation, she says, "I'm an autumn married to a winter and we have a winter son."

Vance, once a model for Saks Fifth Avenue, is a color analyst who believes that physical characteristics like hair, eye and skin color determine the colors one should wear and decorate with.

Vance has a contract with Karastan Carpets, which has adopted her theories in its booklet titled "Color and You." The booklet discusses each seasonal type in detail, down to the fabric, c, wood, metal, decorating style and, of course, carpet, each should have. It was her Karastan contract that brought her to Washington recently where she held color consultations for up to 200 people at several W & J Sloane stores.

Vance's classifications are based on the seasons. Springs, for example, have blonde hair, ivory skin, aqua, hazel or "yellow green" eyes, according to "Color and You." Summers have "soft ash or taupe brown" hair, pink skin with a bluish undertone, and "sky blue" or "rose-brown" eyes. Autumns have reddish hair, and brown or peacock blue eyes and skin which ranges from "beige to copper," while winters have blue-black hair, white to dark olive skin and intense blue or nearly black eyes.

According to "Color and You," summers, for example, should wear mulberry, and decorate in 18th-century, Georgian or Colonial style with crushed velvets, damasks, dark woods and plush carpeting. The bocklet lists a greater range of colors and a lesser range of decorating stylist for each seasonal type.

On the basis of these theories - "I didn't develop them," Vance demurs, "God did, they're [WORD ILLEGIBLE] - she gives wardrobe and interior color consultations. She does not recommend design styles or furnishings.

Vance operates out of her Arcadia, Calif., home-studio. She teaches about nine classes per week and claims to have had several thousand students. Two-thirds of her clients are women who pay $40 for a series of four 2 1/2-hour classes or $75 for a three-jour consultation. She spends Saturdays in individual consultations with clients ranging from Frank Sinatra's decorator to Jacqueline Getty and the Bank of America.

Other clients fly in from Florida or New York. Vance, who's been in the color business since her graduation from UCLA 20 years ago, is currently negotiating a book contract with a New York publishing house.

Diana Vance is clearly a woman for whom color played an important role, even before it was the basis of a theory of lifestyles. Her conversation is sprinkled with unself-conscious references to "the right bright," "the wrong neutral." Of her mother she says, "Her tastes are more primitive because she's a winter."

She even remembers how the wrong colors made her life miserable. "I had to quit one job after six months because I had to wear navy blue ("Color and You" says autumns should wear "neutrals . . . citron, terra cotta, avocado, periwinkle"). I almost had a nervous breakdown. No way was I going to stand around in the wrong colors for a third of my life."

The real challenge of interior color consultation, says Vance, occurs when one must accomodate several defferent seasons under one roof.

The essence of this is compromise, and Vance cites her own dual-season family as an example. "It took 17 years to get a dining room tabke we all liked."

Sometimes it's not so easy. Vance recalled a problem her winter husband had in autumn bedroom she found so soothing. "He had to wear a mask in order to sleep because at one point the color of the walls came right through his eyelids and kept him awake. We later moved, but not for that reason."

Vance is asked to do a color consultation for the interviewer. She begins by asking the interviewer's favorite color. When told it's purple, she bursts out laughing and says, not unkindly, "That's funny, you don't look like a 7-year-old boy." She continues, "Most little boys love purple, but old laddies hate it; they prefer more subdued, muted colors." This is a bit perplexing, since the interviewer is neither a 7-year-old boy nor an old lady.

There are those, Vance notes, who refuse to wear or decorate in their correct colors because they simply don't like them. She say, "I have a summer niece who's married to a summer man and they have a summer daughter. They decorate in autumn and always will. They interpret that as homey."

Certain personality traits, Vance maintains, correlate with seasonal types. Summers, she says, are sweet, serene and easygoing. Springs are unflappable and can "walk out and never look back." Winters are compulsive and methodical. Autumns are nervous and career-oriented, "born leaders," says Vance of her seasonal type.

Jim Harris of Sloane's is asked whether he thinks Vance's color theories are faddish, and whether he feels people will continue to decorate in colors they like, not colors based on their seasonal type. He says, "An individual's relationship to color is timeless, whatever name it goes by. You can fit just about any color, depending on sahde, into a spectrum. (Vance's) theories are basic guidelines, not formal dictates."

Vance says her theories allow for a choice of colors. If you're a summer who hates powder pink there's always azalea. "You can have a preference," she explains.

Diana Vance has a different sort of preference. While noting that "In 16,000 more years we'll all be winters," she says, matter-of-factly, "I'd like to come back in my next life as a spring."