IN WASHINGTON, a city of powerseekers and transients, first impressions count. You are what you wear--at least for the first few minutes.

"I can always tell when I'm in Washington," says image consultant James Gray. "The big, everlasting look here is the Ivy League, preppy look. Only in Washington do you see 12- to 15-year-old kids on a Saturday night dressed in a white shirt and navy blue blazer, khakis and ducks.

"And it goes right through to the grandmother and grandfather. Washington is one of those centers of preppydom."

Gray, himself dressed in a blue pin-stripe suit, says people projecting the power look of the embassies, K Street and Capitol Hill are sending this message: "I care enough about myself. You can trust me with the details of my job."

For women, Gray defines that look as "a neat hairstyle, the right makeup that complements her look, the right well-defined suit that doesn't send perhaps a negative signal--low-cut, or slit skirt--and the same attention to detail, not a lot of flashy jewelry."

Although business is booming for clothing and color consultants--with the trend toward more conservatism and less rebelliousness in outer trappings--many people still prefer to make their own statement. Gray cites as an example the "dynamic man" who always softens his authoritative, three-piece-suit image with a tiny, fresh rosebud in the lapel. Sen. John Warner (R-Va.) also has been known to sport a small rose in suits Gray describes as "spectacular."

The message in what you wear, the "sociologists" of clothes-watching are saying again, may be more than you realize.

Your clothing, says Michael Solomon, 27, an assistant professor of marketing at New York University's graduate school of business, "not only tells other people who you are, but it tells yourself who you are."

"You wear your clothes, and your clothes wear you," says Washington psychiatrist Norman Tamarkin. The first things you notice about someone, he claims, are carriage, presence and clothes.

Depressed people, says Tamarkin, often give off signals to the world by wearing clothes that are too big--as if hiding themselves. "As they start feeling better, suddenly more color comes into their clothes. They walk with a different gait; they start getting clothes that fit them, are more stylish."

When a patient is depressed--and this works particularly with women--Tamarkin sends her to a Georgetown salon to get her hair fixed and makeup done. "I sometimes tell patients to pretend they're feeling good. And to pretend you're feeling good, you have to look good."

Tamarkin can tell when one patient, a man who is manic-depressive, is about to experience a mood elevation: His clothing changes first. He starts wearing cowboy hats, vests, brighter colors, and carrying three pens in his pocket instead of one. A move toward depression is foreshadowed by a certain sloppiness such as an open shirt or a spot on his pants. Tamarkin uses such signs to adjust the man's medication.

Tamarkin, 44, went through an abrupt change of wardrobe himself about six months ago. After 10 years of wearing jeans to work, he started feeling uncomfortable when meeting someone on the Hill for lunch or even going to the bank. He felt people were labeling him as unconventional, and he didn't want to be seen that way. He stopped conducting business in denims.

"It evolved," says Tamarkin, impeccable in navy blue blazer, light blue shirt open at the collar and lightweight gray wool slacks. "It can happen to all of us."

Friends who hold a strong image of us aren't influenced much by our changes in outfits, acknowledges Tamarkin, but if they throw a dressy party and you appear in casual wear, they're likely to interpret that as a slight.

"When in doubt," he advises, "overdress." He sees people dressing up more all around town. "You can never be overdressed anymore, it seems. Now you can wear a tux to a picnic. They just think that you've got something important to do afterwards."

While clothes may indicate mood to a psychiatrist, they tell Gray, 37, an assistant professor of communications at American University and head of a clothing image-consultant firm called Media Impact, how successful a person wants to be. In his workshops, "Occasionally I get the lime-colored leisure suit with a floral-designed shirt. It takes a lot of diplomacy. Are you success-oriented? That's the primary question: Where do you want to go?

"I can't openly say, 'Your lime-colored, polyester suit looks terrible.' I have used the humorous approach: 'How was the vacation in Hawaii?' "

Gray recalls that in the Reagan-Carter debates, Carter suffered with a camera angle that diminished him, but more than that, he remembers Carter wore a tie clasp. "It made him look even smaller. It took in the focus."

Abby Hirsch, founder of The Godmothers Inc. dating service, says she has been observing in clients "an inordinate amount of being able to describe the sort of person they might want through something they might wear." A woman, for example, may say she wants "a Brooks Brothers breakfast-eating man. That's toast, marmalade and The Wall Street Journal. She's pinned a whole life style on that."

Among people with a casual attitude toward clothes, "There's an absolute disdain of designer clothes. They say, 'Why don't they give their money to the poor?' "

Hirsch, in her late thirties, has seen heads of corporations scan a room for a woman who is dressed in a similarly conservative way. They are "unconsciously rejecting the open-toed shoe, anything that looked a little jarring or eccentric. It's being," she says, "familiar with something.

"Clothes," she claims, "go beyond making a fashion statement now. They go into judgments of morality."

For people like Susan Green, 34, an associate professor of psychology at George Washington University graduate school, clothes are to be used pragmatically: She wears suits at the beginning of school to establish authority and then dresses casually later in the year.

For others, the choice of clothes may be dictated more by the subconscious. When a friend of hers showed up one day looking unusually well put together, Green asked her what was going on.

"I had a good dream last night," replied the friend. "And any time I have a good dream, I try to look very spiffy the next morning."