The widow of Washington Colorist Morris Louis will donate 15 of the artist's paintings, drawings and collages to the Fort Worth Art Museum, giving the Texas museum the world's most extensive collection of Louis' work.

In addition, the museum will announce tomorrow, it is in the process of buying another Louis from his widow Marcella Brenner, a retired George Washington University professor of education.

"I chose a city where there weren't very many paintings," said Brenner of her donation. "There was interest, however. Things are happening there. There are a lot of very good bubbles in the champagne down there."

"In terms of one single museum that covers the full scope of the career, Fort Worth has now the absolute top position," said Diane Upright, senior curator at Fort Worth and a Louis scholar.

Louis, who died in 1962, was born in Baltimore and worked for years in Washington, where his richly colored stained canvases made him the best known member of the Washington Color school. Museum officials would not discuss the worth of the donation yesterday. The record for a sale of a Louis painting is $473,000, for a work sold at auction in 1984.

Washington, with a total of 12 Louis works scattered among its museums, falls behind both Fort Worth and the Boston Museum of Fine Arts, which has 16, in the number of Louis works.

"At least in terms of quality, I would argue Morris Louis is certainly shown in this city at his best," said John Wilmerding, deputy director at the National Gallery of Art, which owns two Louises and has a third on long-term loan. "I don't think we've worried about having his work because he is from here. Louis is of interest to us because he was a great painter, not because he's Washington-born."

Referring to the Corcoran Gallery of Art and its emphasis on local artists, Wilmerding said, "We know one museum, a major one, has already staked out that interest."

"I think it is too bad that we and the National Gallery and the National Museum of American Art haven't ended up with more Louis," said Jane Livingston, Corcoran associate director. "Museums do compete one with the other in cultivating these things and you can't always succeed. But there are all sorts of reasons for it to go to Fort Worth."

Fort Worth is currently establishing in-depth collections of major 20th-century artists and in the last year has acquired 20 works by Jackson Pollock and 10 by Robert Motherwell. Livingston pointed out that the Texas museum has the space to promise donors that work will be shown well. The museum also has curator Upright and Director E.A. Carmean Jr., a former National Gallery curator who organized a 1976 Louis show while in Washington. Both have worked with Brenner for years.

I.S. Weissbrodt, legal representative for the Louis estate, said that exposing other parts of the country to Louis was not the only reason for looking beyond the Potomac.

"Boston, for example, manifested a great interest in Louis paintings in the early days, and you know how people react -- it's human nature," he said. "A museum manifests great interest in Louis, and a curator shows great interest, and whenever an exhibit travels they express interest in it -- those are factors in Mrs. Brenner's decision."

The museum is buying the largest painting, "Dalet Kaf," from Louis' series of "Veil" paintings, which marked the turning point of his career. The 15 donated works consist of five paintings, six drawings and four collages, the artist's only known attempts in the medium. While discussing the gift with museum officials, Brenner emphasized the importance she places on her husband's work being seen, not only on the walls of the museum, but in the museum shop.

"I'm interested in having attention stimulated, on the one hand on the part of the serious student, and also with posters and cards," she said. "I think when people go to museums, they like to have a chance to go buy a card and take home a little bit of what they saw."

The Fort Worth Louises will go on display in April. A major retrospective of Louis' work will open at the Museum of Modern Art in New York in October 1986 and will travel to Fort Worth and the Hirshhorn Museum here.