Can Ronald Reagan do for brown what Nancy Reagan has done for red?

Whatever the reason, brown suits, to which President Reagan is partial, are returning to the racks of Washington stores. And that, in spite of the fact that John Molloy, molder of the Dress for Success mentality, calls the brown suit "one of the worst suits made and should be avoided."

Malloy also writes: "It is negative before public and on television. Brown is negative if you are trying to be more authoritative and negative if you are too authoritative."

Even Giorgio Armani, master of Italian tailoring for men and women, concedes, "Solid brown is not flattering, not an elegant color. For an evening dress, that is another matter. A brown evening dress is, in fact, very elegant. If I did evening dresses, I would make it brown for sure." Just the same, Armani made brown suits and brown sports coats this season. "But in a modern way. Never flat brown," he says. And they have been a big success.

Alexander Julian has always included brown. "Brown goes with my red beard, so why not?" he teases. Brown is selling as a color for sports and leisure as well as business. "There is more brown around than in years," says Chip Tolbert, head of the Men's Fashion Association.

Now, after brown, will President Reagan do the same for plaid slacks?