Even a brilliant critic shouldn't try to tell an artist how to paint

I'm disturbed at how clinical much contemporary art criticism has become - and how closely some art critics resemble scientists dissecting specimens in their laboratories.

It's not the analysis I object to, but the implication that art criticism is an exact science - and that anyone who dedicates himself to it will soon know all there is to know about art.

That, I'm afraid, just isn't so. No matter how brilliant or dedicated, art critics - with a few exceptions - are able to know everything about art except the most important thing: how to make it.

That is no small matter, but the issue upon which all writing on art hinges. The best any art critic can do is help shine a little light on the authentic and good, and help prune away the false and bad. In the process, he'll need a system of analysis, but it will be, at best, tentative and imprecise - and always subject to the creativity of the artist.

That's just the way it is. And yet there are some critics and writers on art who think they've beaten the system. Who not only believe they've discovered the truth about art, but that it's their solemn duty to insist that all artists accept it. Fortunately, most won't, and those who do end up the worse for it.

In short, any artist who lets an art critic tell him what or how to paint is either not an artist - or a fool. By definition, the art critic must defer to the artist - not the other way around. It's as simple as that. Key to Rothko: color

Anyone still in doubt about Mark Rothko's quality as an artist should march himself over to the Pace Gallery's current exhibition. He'll find a choice selection of paintings from Rothko's classic period (1948-69) that shows him at the top of his form, as well as a few works that suggest directions he never fully explored. But more important, he'll find a few paintings that are so quintessentially Rothko that even a few minutes in their presence should awaken a significant response - at least to his handling of color, if not immediately to the fuller dimensions of his art.

Color, as a matter of fact, is an excellent key to Rothko's art and an illuminating guide to its significance. Very few artists committed so much of their art to the subtle relationships of two muted colors against a third, or insisted that a painting's identity hinge on the manner in which a band of one color advances upon another.

Color was the cutting edge of Rothko's creative sensibilities, the device that enabled him to evoke and materialize feelings previously considered inaccessible to painting. The process was a slow and gradual one, however, and one over which he only achieved full mastery toward the end of his career. Only then do we feel he knew preciselym what he was doing, that he was no longer fishing for new subtleties and sensations, but was using his art at maximum effectiveness to coax something invisible but profoundly significant down onto his canvas.

He didn't always succeed. In an art as finely honed as his, the slightest miscalculation or lack of alertness could doom a work to failure. That he succeeded at all is a marvel. And that he succeeded as magnificently as he did in a handful of paintings is very close to a miracle.

At the Pace Gallery, 32 East 57th Street, through April 30. The exhibition catalog includes an excellent essay on Rothko's art by Irving Sandler. It's a model of its kind, and should be required reading for all aspiring art critics. The gallery beat

There are roughly 500 art galleries in the metropolitan New York area, and at least 120 new shows every week. Even discounting those in galleries catering to less-than-serious tastes, that still leaves an art critic with a bewildering number of exhibitions to see. He cannot, of course, cover them all, and so must choose among them on the basis of such things as a gallery's reputation, an established artist's performance record, a new artist's word-of-mouth publicity, or a particularly effective exhibition announcement.

My own favorite method is to cover as many galleries as I can in an area noted for its many galleries (SoHo, West 57th Street), and to then write about whatever stands out. I've just returned from such a ''field trip,'' which took me down the west side of Madison Avenue from 79th to 57th Street and then along the south side of West 57th Street to Sixth Avenue. The following are a few of the shows I found particularly interesting or outstanding.

Karel Appel's exhibition of recent paintings at Gimpel & Weitzenhoffer Ltd. is his best in years, and one of the liveliest shows in New York at the moment. There is a passion in these paintings I haven't seen in his work recently, and the imagery is starker and more pointed. His color, also, is lighter and more vibrant. But most of all, it's obvious he's once again taking great delight in the act of painting, and that he's now clicking along at top form.

At Gimpel & Wietzenhoffer, 1040 Madison Avenue, through April 23.

Jedd Garet's show at the Robert Miller Gallery is another story entirely. It consists of several of his typically startling paintings in which rubbery forms; hot, strident colors; and bits of architectural and design motifs combine to create dramatically garish images. The sensibility behind these works is blatant and coarse, and the paintings themselves would have been dismissed as kitsch only a few years ago.

And yet, there's power in some of these canvases, as well as a strange kind of beauty. Garet knows precisely what he's doing and why, and it wouldn't surprise me at all if he manages to pull it off in a year or two. I don't like his work, but I'm impressed by it. Someplace inside all that pictorial hokum is something quite real.

At the Robert Miller Gallery, 724 Fifth Avenue, through April 23.

Hyman Bloom's paintings and drawings have fascinated me as long as I can remember. Although originally an Expressionist with very romantic leanings, he had become sufficiently ''abstract'' by 1951 to be included in Thomas Hess's important book on the leading early Abstract Expressionists, ''Abstract Painting.'' It was a mistake, however, to have included him, for he had never really deviated from his own highly interior, representational vision of art - and he hasn't done so since.

His current exhibition at the Terry Dintenfass Gallery here consists mainly of still lifes in which complexity of composition and exquisiteness of color vie for top spot. Unfortunately, complexity wins out - although there are enough passages of ravishing color to make the show worth seeing. Even so, it's a little heartbreaking to see flashes of genius trapped within several square feet of painterly competency.

At the Terry Dintenfass Gallery, 50 West 57th Street, through April 28.

And last, but not least, I was impressed by the exhibition of watercolors and drawings by Alfred Kubin (1877-1959) at the Galerie St. Etienne. It's a major retrospective of this highly idiosyncratic Austrian draftsman and illustrator, and includes several pieces never before shown in the United States.

At the Galerie St. Etienne, 24 West 57th Street, through May 7.