The fashion business is going to pieces.

Taking their cue from the way women are buying clothes these days, adding a jacket here, a sweater or blouse or other spare part to brighten up what they already own, designers presenting their collections for fall have zeroed in on individual, often luxury separates to spice up customers' wardrobes.

In an obvious bow to the current economy, clothes are mostly familiar and safe, occasionally to the point of being dull, with no drastic changes to put things out of date, no strong new directions in silhouette or hem to make anyone uncomfortable with what is coming up or what they already own.

In fact, only ruffles disturb the calm. Blouses are ruffled, sweaters have ruffles and even wool suits and pants (in the hands of Bill Blass) get the ruffle treatment as a way to make classic shapes look pretty and feminine.

The other indicator for fall 1980 is color. It can be used as minimally as a pair of bright-colored shoes or hose or as boldly as a bicolored dress or coat. Purple of every shade is still the New York designers' passion, although it was rarely seen on the fashion runways in Paris or Milan. Pastels are prevalent, particularly pink, and the brights include red, teal, white and black.

Color is the big spirit-lifter, says Marjorie Schlesinger Deane, president of Tobe Associates, merchandising consultants to top stores. "Today, clothes must not only look good and feel good on, but must make you feel good wearing them," she says.

Among those things designers are counting on to lift their customers' spirits, as well as their own profits, are:

Silhouetes loosened up from their snug fits of a year ago, but not nearly as oversized as in the Annie Hall ear. Blousons, bomber jackets, full skirts, full pants, capes, tent coats and dresses carry out this look.

Less exaggerated shoulder widths, but shaped with sleeves that are rounded and full at the top. (Only Calvin Klein has noticeably broadened shoulders for fall.)

Hemlines holding at just below the knee, though long full skirts are a real option. Sweaters and tunics, shown by some as minis with opaque hose, can be worn with pants or skirts by the not-so-young or not-so-slim.

Sweaters becoming increasingly important for daytime or evening, particularly when handmade, or appearing handmade, whether classic Fair Isles or fragile mohairs and angoras. A tilt to something that hasn't been around for a while -- the sweater, sweater set and sweater dress, obvious winners in an energy-short economy. (Anne Klein, who last year made two sweaters, has 21 for next fal.)

Pants loosening from their straight-and-narrow fit to a fullness that often comes from pleats. Including the classic-length trouser, pants hit everyplace from the knee down except for the extra length of past seasons, which would flop over the shoe.

The first hint that designers were offering pieces of clothing rather than total costumes was in the Halston show when models began exchanging jackets and coats to show off their versatility. A black velvet jacket, for example, first shown with a matching skirt, was then shown with gray flannel trousers, then with an evening costume.

"No one is going to buy anything unless it is totally useful but also irresistible," insists Oscar de la Renta, who is counting on passementerie on suede and wools, bright colored ottoman suits, evening dresses of black velvet and ribbon-trimmed satin to start consumers spending.

De la Renta and others won't have long to wait to see how irrestible their styles are. The first test will be the buyers who are placing their orders in New York this week. And the test that really counts comes in July when this merchandise first arrives in stores.