Those wwo paid close attention to the saris worn by Sonia Gandhi, wife of the prime minister of India, on her recent visit to Washington got a preview of several upcoming, significant textile shows.

Gandhi, who was so careful to chose saris representing many different provinces of India, wore at least six of those that have been specially created for exhibitions at the Musee des Arts Decoratifs in Paris in October, the Metropolitan Museum of Art's Costume Institute in December, and the textile show of the Smithsonian that is part of the ongoing Festival of India. Her saris are duplicates of those being created on more than 6,000 looms all over India for these exhibitions.

Marthand Singh, a dashing bachelor and escort of Jacqueline Onassis, is the prince of Kapurthala and a costume and textile expert who has organized the upcoming show of the Metropolitan for Diana Vreeland. In fact, the 220 costumes have already been shipped off. Still to come are the accessories and the introduction to the catalogue, which Singh is writing while sitting on a houseboat here in Srinagar.

"The Splendours of Princely India" is bound to be the most influential of all the Vreeland shows to date. Fabric pros from the top fashion houses, including Yves St. Laurent and Calvin Kline, have been in India scouting fabrics for their next collections, according to Singh, who is a good judge of such things. The show's bright colors will have an instant impact on viewers (particularly noticeable in contrast to the Metropolitan's current "Man and the Horse" exhibit), as will the return of silver in fabrics and big, bold checks. Even Issey Miyake, who has never used metallic fabrics, has looms weaving fabrics with silver weft and cotton warp, which will make the fabric very soft.

Singh, who is called Mapu by his friends, has been sending Vreeland samples of some of the special fabrics Indian craftsmen have created using techniques of the past thousand years. The ones that have most excited Vreeland and Singh are the brocades in which the background is silver and the colors woven through are like jewels in a fine setting or like beautiful enamels. Vreeland, whose eyesight is weakening, sent a telex to Singh after getting the first samples. "Only now do I realize that brocades and appliques have a beautiful texture, too," she said.

Last week Singh was in a taxi en route to some of the workshops where special pieces are being made for the gift shops at the Metropolitan and Smithsonian. His eye caught the subtle but beautiful embroidery of a Moslem woman's black veil. By the next day Singh had discovered the weavers and embroiderer of this veil and ordered one as a last minute addition to the Metropolitan show. Unlike those worn on the streets of Srinagar, this black veil will be silk and worn with a generous dose of Indian jewelry on the mannequin's head.