Alma W. Thomas, the Washington color painter whose second art career earned her a national reputation, died yesterday at Howard University Hospital following aortal surgery. She was 86.

Miss Thomas, the first person to graduate in art from Howard (in 1924), spent her first career teaching art to students at Shaw Junior High School. It was not until she retired, in 1960, that she began to paint the brightly colored abstract pictures that would win her fame.

Her work was widely exhibited, not only in the galleries but in the White House and in U.S. embassies abroad. In 1972, she was honored with a solo show at New York's Whitney Museum of American Art. In September of that year, when her retrospective exhibition opened at the Corcoran Gallery of Art, Mayor Walter Washington designated the opening "Alma Thomas Day."

She was born in Columbus, Ga. in 1891. Her parents moved to Washington in 1907. Miss Thomas grew up in an old house on 15th Street NW, where she lived until she died.

"The display of the designs formed by the leaves of the holly tree that covers the bay windows in my home greets me each morning," she once wrote. As that statement indicates, nature was at all times her central inspiration.

Her roots were in impressionism. Like many other, Howard graduates of her generation, she was influenced initially by the shcool of Paris painting. At first she painted modest still lifes, jugs of flowers, bowls of fruit. It was not until the 1950s, after studying "creative painting" at American University, and working with Joe Summerford, Bob Gates and Jacob Kainen that her vases and her apples began to flatten into the separate, thumbsize pats of color that would become her trademark.

The early color painters here were a rather quarrelsome group. Miss Thomas was an exception. She was known for smiling at everyone she met.

Although her color paintings in the 1960s, like those of her colleagues, were structured by geometrics, she was not a hard-edge painter. She did not work with masking tape, and though her pats of color often were arranged in parallel lines or radials, their boundaries evoked the palette and the brush.

She once said that her works were not pure abstractions, that she was "painting nature," and the titles that she gave them - "Earch Wrapped in Sunset," "Spring: Delightful Flower Bed," "Wind Dances in Flower Garden," "Late Fall Leaves" - reinforce that view. Her paintings frequently suggest a mood of cheer, of gaiety. Her work was full of joy.

Miss Thomas received a master's degree from Columbia University in 1934. Her first individual show was held at American University in 1958. She also exhibited at the Barnett Aden Gallery, at Howard, and at Franz Bader's here. Her works also were shown in various group shows ("Afro-American Artists," "Washington: Twenty Years." "Contemporary Black Artists in America") at museums in Baltimore, Boston, and New York.

Because many in the art world feel that works by blacks and women have not in the past received the attention due them, and because many people are now working to right that situation, the works of Alma Thomas - a black woman Washington color painter - are certain to be closely studied in the years to come.

Miss Thomas recently arranged to leave a choice selection of her paintings to the Smithsonian Institution's National Collection of Fine Arts.

She is survived by a sister, J. Maurice Thomas, of the home, and a niece, Alma Louise Holcomb.