Orange tomatoes. Purple broccoli.

Heeding dieticians' message that a rainbow of colors offers balanced nutrition, plant breeders have begun bulking up some traditional fruit and vegetable selections with still more nutrients.

"Before, they were breeding for higher yields, stronger plants, produce easier to ship and more ornamental in appearance," said Grace Romero, lead horticulturist with W. Atlee Burpee & Co., America's largest home gardener seed company.

"Now they're looking at improvements in flavor and smell in addition to more nutrients. Enriching the colors is attached to nutritive value."

Many naturally occurring plant pigments also are the stuff of disease-fighting phytochemicals and antioxidants: concentrations of carotenoids, for example, which give fruits and vegetables distinctive yellow, orange and reddish hues. Or anthocyanins, which give strawberries and beets their trademark tones.

"The most nutritious colors are the rainbow colors," said Lilan Cheung, director of health promotion and communication with the Harvard School of Public Health. "Nature has endowed each fruit and vegetable variety with certain vitamins, minerals, fibers and more."

No single food contains all the necessary nutrients, but different plant color groups when eaten in combination fulfill the average person's daily requirements.

"Orange and green, definitely, we should have something from these groups every day," Cheung said. "Purple or blue, dark green and orange. Reds don't need to be part of the daily diet mix but they should be eaten frequently.

"People should be eating nine or more servings of fruits and vegetables per day," Cheung said. "The more, the better."

We're not talking just fresh fruits and vegetables. Nutrient levels also are high in produce that has been canned, frozen, dried or processed into juice.

Burpee is working in particular on elevating their tomato, broccoli and pepper products, Romero said.

"Tomatoes with more yellow and orange, although reds are still the consumer favorite. Research papers show orange tomatoes have more vitamins than red tomatoes," she said. "Something purple would have more anthocyanins than something that's white. Purple broccoli, for instance."

Here are some rainbow groupings, along with their disease-fighting capabilities:

— Red: Tomatoes, beets, radishes, red bell peppers. Lycopene, found in tomatoes, is known to reduce prostate cancer risk.

— Green: Spinach, broccoli, brussels sprouts, cabbage, green peppers, kale, collard greens, peapods, asparagus. "Phytochemicals lutein and zeaxanthin are found in spinach, collards, kale and broccoli," Cheung said. "These antioxidants may protect the eyes and fight against free radicals — harmful substances in our body caused by smoke and pollution."

— Yellow and orange: Carrots, pumpkins, yellow peppers. Especially rich in beta-carotene, which strengthens the immune system.

— Blue and purple: Eggplant. Loaded with antioxidants called anthocyanins that may prevent heart disease by blocking the formation of blood clots.

— White: Garlic, white onions. "These pungent vegetables add flavor to foods so you can cut back on the salt," Cheung said. "The onion family contains allicin, which has anti-tumor properties. Garlic is being studied for its potentially beneficial role in preventing heart attacks and strokes, although the research is still preliminary."