For many years, the closest the average cook came to an exotic combination of spices was McCormick's chili powder.

But here's good news for Mexican-food fans: the pepper picture is changing. As supermarkets in the Washington area begin to explore territory previously claimed by obscure ethnic groceries, it becomes easier to find a variety of fresh chilies in neighborhood markets.

Choosing among them, however, is another matter. Even the well-stocked produce counter may lack adequate descriptions that would help the chili novitiate decide on the proper purchase. The rainbow of colors, the variety of shapes and sizes give no real clue to the punch lurking within. Odonna Mathews, consumer adviser for Giant Foods, says that Giant plans to label the peppers in all stores as soon as it devises a suitable sign. Safeway's plans, says consumer adviser Barbara Beizer, are similar but less definite.

The problem, says Karen Caplan, vice president in charge of sales of Frieda's Finest/Produce Specialites in Los Angeles, could be solved by packaging the peppers and labeling each container with appropriate identification, personality profile and recipe suggestions. Bulk sales are fine for common foods, she says, but not for such confusing and unfamiliar foods as chili peppers.

Varieties number in the hundreds, and even peppers of the same variety may differ in flavor, according to the soil in which they are grown. But Caplan offers a basic rule of thumb: "The smaller and darker the chilies, the hotter they are."

It's a rule to be loosely interpreted--nearly every pepper turns red as it matures. But it fits most of the peppers sold in this area. The hottest--serrano chilies--are tiny and dark green. And it can get confusing. Very large, dark green peppers (anaheim) are mild, while long, green peppers (finger or cayenne) are extremely hot.

Once chosen, peppers should be handled with extreme caution. Like the palate, skin and eyes can be burned by the strong irritant which gives peppers their heat. Mexican cookbook author Diana Kennedy recommends that people with sensitive skin cover their hands with thin rubber gloves before preparing peppers. Anyone working with chilies should avoid touching his eyes, nose or mouth until he has washed his hands with soap and warm water. Also contact lenses should be removed before pepper preparation.