Howard Mehring, the Washington Color Painter, is - correction: used to be - a singing, unpredictable, important abstract artist. His long-awaited retrospective at the Corcoran Gallery of Art is a triumph without joy. The saddest thing about it is how suddenly it stops.

For reasons that he will not, perhaps cannot, state Mehring has not made a painting since 1968.

Ask him who and he evades. He is only 46 now, a soft-spoken, bearshaped man who in conversation seems both eager and suspicious. Will he ever paint again? "Sure," says Howard Mehring, as if there were no doubt. Then why is it he stopped? The moment is uncomfortable. Mehring starts to press his arms against his sides as if diminishing his size.

"Well," says Howard Mehring. "First I lost my studio. If I could find another, I'd start again right now." He tries another tack. "The paint," he says, "the paint fumes really got to me." Suddenly his eyes flash as if in anger. "Look," he says. "It's a long story. I'd rather not go into it. Let's accent the positive. OK?"

Mehring's show begins in 1957. It is finest at the start.

The large stained hard-edge painting of the Washington Color School today are out of favor. When new they seemed amazing, baffling, prophetic - the past had been glimpsed, progress seemed inevitable - but that mood of soaring optimism did not linger long. Too many imitations, too much effusive praise, and a fading of their newness would soon make color paintings, the lesser ones at least, seem too soulless, easy, pat.

In 1957-58, when Mehring painted "Equinox," "Playground," Frontenac and "Snowfall" - the extraordinary pictures with which his show begins - a similar shift in fashion had begun to dim the promise of the New York School.

The anguish of the action painters no longer seemed heroic, their pictures had begun to feel too dark, too mean, too messy. Cool was coming in frenzy going out, when in 1957 the Washington color painters together made their move. Their paintings weren't occluded; instead they opened up the field. They fought messiness with strict geometry and darkness with bright color.

Of all the Washington color painters except, perhaps, for Morris Louis, Mehring was the one who best understood the power of free gesture. His early works on view with their calligraphic brushstrokes draw much of their beauty from his wrist, his hand. That is among the ironies of the Mehring retrospective. Although he later turned to masking tape and straight edge, he was at his best when he improvised and drew.

Mehring learned much from his teacher Kenneth Noland and from Jackson Pollock - one sees that in his work - but he also learned from jazz, from Charlie Parker, Thelonius Monk, Miles Davis, Eric Dolphy and Lee Konitz. Mehring's art is most impressive when it is most free.

That may partially explain why in 1968 Mehring stopped painting. His ruler blocked improvisation, the hard edge cut his freedom. One feels a growing tightness, a lessening of liberty, as one moves through his show.

Perhaps he understands that now. Although he no longer paints. Mehring once again has begun to draw.

A selection of his recent drawings, all done with felt-tipped pens, are on exhibition at the Philips Collection here. They are modest in scale and timid in color, but at least they are not chained.It is clear they were done quickly and without inhibitions. Connoisseurs of Mehring who believe that he is finished ought to see this show, for in it he returns to the spontaneity - one is tempted to say happiness - that so enriched the paintings he made 20 years ago.

There is something almost morbid about the mood evoked by the Corcoran retrospective. Some painters draw more nourishment from their minds than from their hearts, but Mehring is not one of them. One can almost feel the painter battling his gifts, fighting his won instincts, as one moves through his show. His works retain their quality, but at increasing price.

In 1957-58 Mehring, as he painted, drew with confidence, with ease, but then, for reasons of his own, he lost faith in his brush. He lifted it from the canvas and instead of drawing on the cloth began painting from a distance, dappling the canvas with tiny staining dots of thinned Magna paint. Mehring's rich all-over field paintings of 1959-60 ("Orgone Eminence," "Gold") are radical and daring. Also they are beautiful. But once Mehring had begun to move calligraphy towards rigor there was no turning back.

He then began to cut rectangular shapes of dappled canvas for his collaged paintings of the early '60s. With these striking pictures one feels the closing-in begin. He had fallen for the edge.

It may well have been the influence of Noland, or it may have been the spirit of the time, at any rate the painter would soon abandon drawing. In the last works in the show, the so-called "Upside-down Ts" and "Zs," there is no trace at all of Howard Mehring's hand.

Their color saves these paintings. It is like no one else's; it is odd, disturbing. Look, for instance, at the way that Mehring uses black, pea green, bright orange-red and yellow in the "Upside-down T" called "Nova," a work from 1965 that was purchased when still new by the Museum of Modern Art. "In some paintings," said Mehring in 1962, 'I use harmonious colors to create a balance theme; in others I try for resonance; in still others - try for a dissonant note. I often deliberately try for sweet effects, for lyric expressions of color just because they are a challenge. It is hard to paint pretty colors without getting too sweet. I like unexpected effects, lavender with orange, like a sudden discord in music."

In the last workd at the Corcoran, the colors are all "pure," flat and uninflected. "Pure color" was a battle cry of the Washington color painters.

It well served Kenneth Noland, whose art is diamond hard, and it also served Gene Davis, who rarely tried to draw, but Mehring gave up something when he abandoned drawing. Then, in 1968, he abandoned more than line.

The color paintings of the late Moris Louis now fetch prices in six figures. Early works by Noland are also very dear. Those of Howard Mehring aren't. Louis may be the best, the most important of the Washington color painters, and Noland may be second, but there is no giant gap - except, of course, in price - between their works and Mehring's. Howard Mehring, when he painted, was very good indeed.

His retrospective is accompanied by a first-rate catalog with 13 color plates. Jane Livingston of the Corcoran, who beautifully installed the show, wrote the catalog essay. Mary Swift provided a thoroughly researsed detailed chronology.Mehring's retrospective closes Jan. 22.