There are now on exhibition here 15 pictures by Thomas Downing -- the last of the hard-edge Washington Color Painters. Though Downing is still painting, he shows his works but rarely, and none of these is new. Five are at the Phillips Collection. Ten more are at the Addison/Ripley Gallery, behind the Phillips at 9 Hillyer Ct. NW. The most recent was completed 15 years ago.

Together they deliver a strong jolt of nostalgia. It is their warmth that most surprises. Their colors are as strange, as idiosyncratic, as oddly tart and bold as one had remembered. But their harshness has diminished. They now feel like old friends. Their circles are still round, their color planes still flat, their geometries still strict, but their rigors have been softened. How handsome and eye-pleasing -- one might almost say how pretty -- these once harsh and shocking paintings have started to appear.

His colleagues liked to add to the range of their methods. Their repertoires kept growing. The late Morris Louis liked inventing art techniques that no one else could master (think of his "Bronze Veils"). The late Howard Mehring liked the gestures of the brush. The late Gene Davis, too, kept searching for surprises, for new whimsies and new grandeurs. But Downing rarely splurged. Each time he began doing something new -- when he added to his palette a dozen shades of yellow, or made his disks bright white, or began exploring isometric perspective -- one felt that he was searching for something to get rid of. In his later works he's been content with a single color only -- a brilliant cadmium red, which he occasionally dilutes to a fleshy, somber pink. Rare, indeed, the colorist content with one color. Downing's faith has never wavered. Now approaching 60, he is one of the last Minimalists. He believes his art grows stronger, that its intensity increases, as he diminishes his means.

His paintings rarely look quite right when seen beneath the track lights of windowless museums. Carefully adjusted, artificial lighting somehow makes them freeze. Look, for instance, at "Dream Rate," a grid of 1962 in the collection of the Phillips. Its smaller background circles are soft pink and baby blue. Its larger foreground disks, most of them at least, intensify those hues -- some are bright blue, some bright orange -- but then he breaks the order by adding circles of an unexpected khaki.

At the Phillips the other day, while few viewers were in sight, a museum employe agreed to change the light. As the rheostats cut in, as the light grew dim then bright, "Dream Rate" started breathing. With every tiny lighting shift it became a different picture. "Tsivory" (1972), a vertical work displayed nearby, also started dancing. At dusk, when portraits vanish, Downing's abstractions start to loom. They are not the same in winter light as they are in spring. A picture of colored circles might sound somehow dumb, but Downing's colors are so precisely balanced, and so eerily responsive to the light that falls upon them, that they seem alive.

Downing often tuned his colors until they played tricks on the eye. Look at the dark blues -- and the sudden jarring yellow -- in "Blue Spring" (1964), now hanging at the Phillips, or at the way he juxtaposes white and pink and khaki in "Fold 5" (1968), a particularly strong canvas at Addison/Ripley. These hues seem poised to jump, and as the light changes, they dance. It is that sense of movement, that sense of animated color, that gives his art its life. The Addison/Ripley exhibition closes June 27. The small show at the Phillips runs through Sept. 8. Simon Gouverneur

Geometrical rigor is not dead. The intricate new paintings of Simon Gouverneur, also at the Phillips, show that vitality can still be found in the old hard-edge tradition.

Gouverneur's art is never loose. He relies upon the T-square, the straight edge and the compass. His work is full of arcs, triangles and circles, rectangles and rings. But unlike, say, Tom Downing, he does not seek the sparse. He likes twinklings and spins and complicated rhythms. There are 654 small, nine-pointed stars -- of silver, gray and gold, white, pink, orange, green and black -- in his "Constellation" (1985). The distribution of those colors probably obeys some predetermined law -- at least no chaos is apparent -- but those complicated laws are too much for the eye.

The Larry Poons-like painting that Gouverneur calls "Formatives" (1985) displays hundreds of small, bright red circles on a flat white ground. Their distribution suggests obedience to some music heard by the painter only.

These and other paintings here -- for instance "Ebb-tide-flight" (1985), a complicated dance of bright blue disk segments and semicircles -- might produce most handsome fabrics. The way they're painted is unimportant. If they were made with cutout papers glued down to the canvas, they wouldn't be worse -- or better. The artist's hand is largely irrelevant. It is only his designs that matter.

They offer some surprises. The vertical lines that march across "Skyline" (1984) imply Egyptian columns; there is, indeed, a teepee in "Tihu -teepee" (1985), a crescent moon glows darkly in "Isomorph," and butterflies and scarabs unexpectedly appear in "Semes" (1984). Simon Gouverneur uses mathematics to fight his way toward myth.

A New Yorker trained in Spain, he is a former professor at Howard University and is currently artist in residence at the Maryland Institute, College of Art, Baltimore. His show closes Sept. 8.