Studies in black and white

Nothing looks so clean and well-defined as black and white, which is one reason we are seeing so much of it this year when purity and simplicity are vaunted assets in dress.

The Americans and the French are both strong backers of this unbeatable classic combination. Except for some delicate pastels, Ralph Lauren centered his entire collection on black and white linens. Some of the snappiest suits around are Oscar de la Renta's studies in contrast: a white peplum and crisp revers on a fitted black gabardine, for instance. Others found graphic punch in play-offs of black and white stripes against polka dots.

Paris designers moved on from the somber chic of that city's all-black winter (black and more black having been the ultimate for the fashionable French this year) to dashing prints of black on white grounds. Ungaro's designs are especially memorable in the spring couture collections.

The return of the ''little black dress,'' a fashion legend, is upon us, but in greater variety than before. Tank-topped cotton jerseys as well as tailored coat-dress styles are part of the assortment. The new jewelry to wear with the LBD is apt to be of ivory combined with jet in smooth-surfaced, art-deco-inspired designs.

Then there are the new black jewelry settings of darkened metal and the all-black timepiece, a status object popularized by European men. Now versions of the black-faced diver's watch with the linked oxydized steel wristband are cropping up everywhere on both men and women.

Black dominates for the moment - it is even the ''in'' color for luggage. But white with sharp black accents has a big fashion future. There will be nothing basic-looking about the ways the combination is worn: a white suit, for example, snapped up with a black patent belt, handbag, and sling-back pumps will definitely make a positive statement.