"The Paper Cut-Outs of Henri Matisse," an exhibit of the bold, brightly colored works which increasingly absorbed the attentions of the master during his last years, will open here Sept. 10 at the National Gallery of Art.

Among the works displayed will be five important cut-outs from the early 1950s which the Gallery purchased in 1973 but has not yet shown.

Matisse, who was born in 1869 and died in 1954, was a painter and a sculptor and a colorist of genius. The cut-outs he began to make with scissors and hand-colored paper when he was in his 60s manage to combine drawing, painting and shaping. In them one detects the magic of his simplifying eye.

Although it will open here, this fall's exhibition is not a Gallery project. It is being organized by two other institutions, the St. Louis Art Museum and the Detroit Institute of Arts.

The Gallery thus is benefiting once again from the curatorial efforts of other art museums. "We're opening in Washington," explains Jack Cowart of St. Louis, "because it's a world-class show."

Matisse would often say that when he cut into pure color, he felt like a sculptor carving stone.

He used hand-painted paper and long-bladed shears. He said that cutting let him draw not with line, but color. "Instead of establishing a contour, and then filling it with color - the one modifying the other - I draw directly in color . . . This guarantees a precise union of the two processes: they become one."

Children cutting paper dolls, 19th-century portraitists who cut silhouettes and many other artists had explored the paper cut-out. In the 1930s, Matisse began cutting sheets of paper while composing paintings. While bedridden, in the last years of his life, he explored the paper cut-out as an independent art form. Matisse would cut his paper shapes, some of them enormous, and then have his assistants pin them to the wall.

"The Swimming Pool" from New York's Museum of Modern Art will be the largest work displayed. Matisse composed that 54-foot mural for his dining room in Nice.

Other cut-outs shown will be borrowed from the new Pompidou Center in Paris, as well as from collections in London, Amsterdam, Basel and Japan.

Between 50 and 60 works will be included in the show. Among these will be examples of his famous Blue Nude series, and cut-outs that he made in the process of designing his Chapel of the Rosary in Venice.

The National Gallery owns five Matisse cut-outs, "Large Composition with Masks" of 1953. "Beasts of the Sea" of 1950, "Venus" of 1952, "Woman with Amphora and Pomegranates" of 1952, the "The Negress," also 1952. The largest of these, the "Large Composition with Masks," is 11 feet high and more than 32 feet long. All five were bought in 1973 with funds provided by the late Ailsa Mellon Bruce.

After leaving Washington, "The Paper Cut-Outs of Henri Matisse" will travel to Detroit and St. Louis. It will be on view here from Sept. 10 until Oct. 23.