California designer LaVetta King got her first clients by knocking on backstage dressing room doors. It worked - landing Lena Horne, Diana Ross, the Supremes, Nancy Wilson and others as personal clients, so the designer, who prefers to use her first name only, figured the direct approach was worth trying on Rosalynn Carter. She didn't expect Mrs. Carter to become a client but only wanted the First Lady to view her designs. Besides, LaVetta's father, Benjamin Forbes, head of the Democratic party in the Orleans parish in New Orleans, had urged her to contact Rosalynn Carter. This week, while LaVetta was at Elizabeth Arden here, she called the White House, but was told to send her things over by mail.

Several years ago LaVatta King supporter Marge Champion brought the designer a shabby scarf dress she had found in London. Intrigued by the idea of scarf-dressing, LaVetta tested her designs first on five-and-dime quality scarves. The current collection of scarf designs, made mostly from Oriental silk squares in flowered, geometric and paisley patterns, now at Elizabeth Arden, are 98 per cent handmade. All hems are hand rolled, buttonholes hand bound, seams hand stitched. Elizabeth Taylor, Nancy Wilson, Alexis Smith and Helen Gurley Brown are among the LaVetta King scarf dress collectors. Scarves are not only used as dresses, but form tunics worn over pants as well. Prices start at $260.

"My dream is to go the White House to put on a fashion show," LaVetta admitted.This is the first season she has done short dresses, which she thinks are well-suited for the White House. "The lifestyle of the Carter family is very homey. They make you fell like you are going to a friend's house."

LaVetta, who characterizes herself as the rare black designer who runs and owns her own business, says black designers were ignored by retailers until five years ago "when they were needed to give a shot in the arm to a dying Seventh Avenue (garment industry). There's really no future for black designers in the fashion industry unless they are owned and operated by a large conglomerate," the designer said. "Without it there is no hope for a black designer to get coverage and support from the national magazines that is needed to get stores to buy their clothes."

Now it seems to be the menswear industry's turn to nudge the first family. The Clothing Manufacturers Association of the USA sent a letter to President Carter indicating that their board members approved of the fireside chat but "were sensitive to the fact that you chose not to wear a business suit while on television, which they feel would have enhanced further the image of the man elected to the highest office of our land." The group applauded the President's taste in buying and wearing made tailored clothing.

Presidential Press Secretary Jody Powell said there were a number of complaints that the President's garb was too casual during the so-called "fireside" chat but "we also got one that wanted the President to wear a tuxedo." That suggestion came form a formal wear store. Powell said.

Two day-long seminars on fashion are scheduled for next week. On Tuesday, the Washington Fashion Group and the Smithsonian Associates' program will include, as speakers, designer Gil Aimbez, the designer for Genre; Stan Herman, who among other projects designs the uniforms for MacDonald's, the five winners of a local contest for college designers, plus textile, cosmetic and retail experts. It will be held at the Smithsonian's museum of History and Technology.

"Blacks and the Fashion World" is the theme of a fashion seminar and workshop scheduled for next Satruday at the Hyatt Regency Hotel. Featured will be designers Lepenski and Edouard; charm school president Precola De Vore; models Loretta Meredith and Marlene Odette Daniels and others. (For more information call 291-7662). There's a charge for both programs.

Washington designer Ann Stuart is among the 12 young fashion designers taking part in the Hong Kong Ready-to-Wear festival which opens today and includes a competition to be judged by Rudi Gernreich, socialite desiger Charlotte Ford and executives from Hong Kong's $3-billion clothing export industry. Stuart, who studied in Paris at the Studio Bercot, designs sportswear for Birds, a chain of boutiques in Hong Kong and knitwear for the English firm and Jeffrey Rodgers.

Stuart makes most of the knitted garments herself by knitting, crocheting, embroidering or sewing each one before it goes into production. "It's fun to be able to design fun clothes at reasonable prices," Stuart has said. "I don't think you have to be rich to dress well."

The Hong Kong festival precedes the fall showing in Italy and paris by several weeks and gives buyers the very first glimpse of new fall styles. Among those to be shown are American Indian-inspired fringed suede tunic worn over a skinny skirt or blouson worn over tights by Virginia Lai.

"If your jeans are clean and pressed, then with a jacket and sweater they can be acceptable clothing even for going to the theater," advises not the principal of a local school referring to the current dress code, but Russian designer Vyacheslav Zaitsev, in Moskovsky Komsomolets, the paper of the Moscow Communist Youth League. Zaitsev, the head designer at the All Union House of Fashion in Moscow, added, "But here it must be noted: (Jean are acceptable) to a youth or student theater, please, but not to the Bolshoi."

According to the newspaper, the Western-imporved jeans fad has caught on, and American-made jeans sell for as much as $66 on the black market, and jeans and matching jacket for twice that amount. (American made-jean are also the rage in Hungary. According to the Communist newspaper there, young Hungarians are paying up to $80 for a pair of imported jeans. Apparently there is a factory in Hungary making jeans according to Levi Strauss specifications which until now made them only for export.)

Zaitsev, who has admitted that jeans are universal and "there is nothing to replace them," has added, "you must observe the esthetic norms - slightly worn jeans are a completely understandable thing, but if they're worn down to holes and patches and are shiny on protruding places, this is already bad taste." He added, "The most important thing is to remember that there is a place for everything and what you wear to go hiking in shouldn't be worn when you are a guest in someone's home. An attractin to jeans should not grow into jeans-mania."