"Washington needs a jot of color," said Mame Cohalan, former actress turned muralist, who is giving Washington a bold blue, mauve, red and yellow jolt right on Pennsylvania Avenue.

With the help of 10 high school students, working under the auspices of the Southwest Neighborhood Planning Council, Cohalan is painting a large mural on the fire wall of a vacant store between 7th and 8th streets, where Kann's department store used to be.

Elsewhere in America, this once-deteriorating area opposite the grim old National Archives building would undoubtedly be an unpaved parking lot or a messy heap of litter and weeds. But it seems we in Washington are beginning to care about our old buildings. Some bureaucracies, such as the Pennsylvania Development Corp., and District planners are art commissioners, have caught this new urban spirit. They seem to care, too.

So the Kann's site, officially known as Market Square, is a neat little park on the way to becoming a temporary art gallery. A few years ago, poor Winfield Scott Hancock, on his bronze horse, looked quite out of place among the porno joints. Not wht porno joints look out of place in this attractive setting.

Both the park and the art are temporary.

In three years or so, the Pennsylvania Avenue Development Corp. hopes to replace the park with a residential complex of 750 apartments. The corporation wants people to live on the avenue. And it wants the building to form a kind of gateway to a Market Square mall, a strong lively axis between the National Archives and the National Portrait Gallery.

The art displays on the lawn, as well as Cohalan's mural in the background, are alos temporary in spirit. Cohalan believes that it is inhibiting and pretentious to attempt a city mural for eternity. And Mike Shafer's sculptures "Gridations," one of several sculptures to come, is a composition of tubes, skillfully fastened together by wire. They can be just as skillfully unfastened.

The sculpture show, entitled "City Art '79," is a welcome indication that the city's art administrators are at last beginning to do their jobs, which was hardly the case under the previous city administration.

The buzz word for all this city art is "temporary," which seems to me part legacy of those silly "happenings" of the '60s, and part admission that all of us are caught in an accelerating whirl of changing fads and fashions in art and tastes. So whatever it is we like and create turns out to be temporary anyway.

Mame Cohalan, for instance, was meticulously dribbling abstract expressionist supercanvases only a few years ago. When the survival instinct (three small children!) as much as artistic Sturm und drang led her to decorate bathrooms for affluent friends, she spared us candy-colored Peter Max sunsets, thank heaven, but painted the then obligatory super-graphics.

The tough of more super-graphics -- if you have nothing to say, say it in giant Helvetica letters and shrill stripes and arrowheads -- makes the idea of temporary art most attractive.

Having gotten striped walls out of her system -- they decorate not only bathrooms, but such diverse places as the British Airways Concorde lounge at Kennedy Airport in New York and the reptile house at the National Zoo -- Cohalan "discovered the Renaisance," as she put it, just as Leland Allen, the Pennsylvania Avenue Development Corporation's in-house architect, discovered Cohalan.

With the help of the D.C. Recreation Department and other agencies, Cohalan and her group of students got their chances to celebrate the renaissance of Pennsylvania Avenue with her mural of Renaissance stage set of colorful colonnaded arches and pointed perspectives.

The Recreation Department pays the studenst minimum wage of $2.90 an hour for this summer job, which is now turning into autumn labor. "Three or four more days of good weather," says Cohalan, "and we'll be finished with the first wall."

Then there are still two successively higher, vacant walls of the buildings behind, on which Cohalan wants to continue her architectural theme. "I want this to be a king of pop-up mural, like the pictures that flip you when you open some children's books," she explains.

Far from Peter Max's psychodelic kitsch and supergraphic deadpan, the zeitgeist moved Cohalan to a creation that I think most people will really enjoy. They wil enjoy her architectural design because these columns and arches strike memories and associations in all of us. They evoke history. aThey are not Rohrschach inkblot abstractions.

In fact, in the Walters Collection in Baltimore, there is a lovely painting of an imaginary, ideal Renaissance city, painted about 1500 by an unknown Renaissance artist, that strongly resembles Cohalan's mural. A parallel painting is in Urbino, Italy, the pearl of Renaissance architecture.

Cohalan said she had never seen this painting. I find this encouraging. It may just mean that she and the other graphic artists who are suddendly making architectural motives a fashionable decoration are not copying but re-reacting.

And that may just mean that the temporary mural on Pennsylvania Avenue is so enjoyable not only because it adds color,not only because it gave some kids a nice summer job, not only because the bureaucrats are doing something nice for a change, but it may also mean that this Renaissance mural heralds a real renaissance of the American city.