Bob Hale took a risk a decade ago, pulling up his yellow onions and planting red ones instead.

"Color is the new thing," he said. He was ignored by the others farmers here at the heart of the nation's onion belt.

But Hale was proved right in 1997, when Pizza Hut switched from yellow to red onions on all its pizzas. Two years ago, Subway, the nation's largest-selling sandwich chain, embraced reds, saying they added a splash of color to subs.

Now, large and small chains are experimenting with the brightly pigmented onion, a highly temperamental plant that takes far more skill to grow than its yellow cousin.

Of the onion market nationwide in 2004, 88 percent was yellow, 7 percent was red, up from 5 percent five years before, and 5 percent was white, according to the National Onion Association.

Fast-food chains are discovering what chefs have long known -- visual presentation is almost as important as taste.

"Consumers, first of all, eat with their eyes," said Shirish Mehta, chief food innovations and technology officer for Dallas-based Pizza Hut Inc.

In the company's pizza lab, researchers were bothered by the fact that the yellow onion blended with the cheese. "Cheese is light in color, and so a white or yellow onion doesn't show up," Mehta said.

So they did a test, putting two pizzas in front of customers -- identical except that one was topped with red onions, the other with yellow. Overwhelmingly, their subjects chose the more colorful pizza, even though the two onions differ in taste, with the red generally thought to be milder.

The decision to switch was a "significant investment" for the chain because red onions cost more than the yellow, he said.

The same held for Subway, which changed to reds in 2003. In spite of the higher price, the switch was a "no brainer," said Subway's Nick Hauptfeld, manager of new product development.

After tests in selected stores, Subway researchers concluded that their customers chose red onions over yellow 3 to 1, he said.

"Red was outpacing yellow to the point where there was no point in having the yellow anymore," said Steven R. Sager, who owns a franchise in Palm Beach Gardens, Fla., where the two were tested side by side.

Last year, Burger King began using red onions in salads, though it is staying with yellow on their Whoppers.

Not all major chains have jumped on the red bandwagon. Domino's Pizza is sticking with yellow after testing red onions and finding it was harder to assure quality and consistency.

The red onion's sudden popularity caught the farming industry off guard, with many farmers forced to scrounge for seeds.

Dan Miyasako, 44, inherited his father's farm in Homedale, Idaho, where the elder Miyasako had been growing yellows since the 1940s. In the last two years, he has doubled his area of reds from 30 acres to about 60 acres, hoping to meet the recent demand from grocery chains.

When he is not out working his fields, he is on the phone looking for a good red seed.

"There's a real shortage of it. It's hard to get your hands on it. You can get the ugly, the not-so-pretty reds. But the real pretty -- the red wing, salsa, red bull seeds -- those are hard to find," he said.

Because the yellow onion has been by far the most popular variety for decades, seed breeders focused most of their attention on yellow. And producing a good variety takes an average of about 10 years, said Ton van der Velden, U.S. sales manager for Nunhems USA, the largest onion seed supplier in America.

"You can put a red in the ground," warned Hale, whose red onions are used in Subway's 20,000 North American franchises. "But you may not harvest it."

Bob Hale of Hermiston, Ore., switched from yellow to red onions some years ago, anticipating a shift in demand.