SURELY a big reason for the Folger Theatre Group's great success with "The Merchant of Venice" is its design concept.

Lunching at a local gallery, artistic producer John Neville-Andrews spotted the work of Mersad Berber, a Yugoslavian painter who has been making his mark in Europe with his strange, dreamlike oils of grave infantas, charging horses, turbaned men, his Velasquez-like figures and dark, rich colors delicately laced with gold, reminiscent of Klimt in their suggestion of wealth and decadence.

"The images, colors and unusual textural content of his work captured my feelings about the play exactly," the producer writes. "The paintings also contain colors for both the hard, businesslike world of Venice and the warm, music-filled Belmont."

Even some of the character concepts apparently were influenced by Berber's faces, which often have the intensity of portraits, from his turbaned Eastern merchants to the homage to Liv Ullmann.

Costume designer Bary Allen Odom and particularly Claudia Rummel, the specialty costume painter, have virtually reproduced some Berbers on the costumes. In fact, the women's dresses have been extended two feet on each side to create a sandwich board effect which -- amazingly -- works very well indeed. On these surfaces appear the rosettes, the cabalistic signs and numbers, the gold filigree and other wonderful conceits that help give the whole scene a feeling of some remote, mysterious, opulent past.

The men's costumes also drift into fantasy with their gold knee rosettes and waistcoats, a dramatic contrast to the simple robes of Shylock (played with intelligence and passion by Richard Bauer as a man so eaten alive by hate that his fingers must constantly rush up to hide the twisted mouth).

Russell Metheny's sets repeat the patterns and colors, adding an Escher-like spiral staircase at the side, a useful variation on the Elizabethan design.

And those colors! They are the colors of an autumn landscape at dusk: plums, burgundies, aquas, ochres, apricots, burnt siennas, dank greens and heavy chocolates, all enhanced by the fabrics, suedes, velvets, silks, corduroys, brocaded and bejeweled and touched with lace. The women's hats are a treat, adapted straight from some Berber paintings. They are bourrelets, broad rolls that go over the top of the head like a pharaoh's crown, built of fine wire screen, gilt and glitter.

One would hesitate to recommend seeing a play just for the costumes. To the contrary, as it happens, this "Merchant of Venice" is a brilliant production, funny and moving and actually believable, making ironic sense out of a situation that modern audiences have trouble taking seriously.