TARRYTOWN, N.Y. - Mozzarella cheese at Panera restaurants won't be as glaringly white. Banana peppers in Subway sandwiches won't be the same exact shade of yellow. Trix cereal will have two fewer colors.

Food makers are purging their products of artificial dyes as people increasingly eschew anything in their food they don't feel is natural. But replicating the vivid colors Americans expect with ingredients like beets and carrots isn't always easy.

In fact, General Mills couldn't find good alternatives for the blue and green pieces in Trix, so the company is getting rid of those colors when the cereal is reformulated later this year. The red pieces - which will be colored with radishes and strawberries - will also look different.

"We haven't been able to get that same vibrant color," said Kate Gallager, General Mills cereal developer.

The shift away from artificial dyes represents the latest chapter for food coloring in the U.S., which has had a rocky history. As recently as 1950, the Food and Drug Administration said children became sick after eating an orange Halloween candy that contained a dye. The agency eventually whittled down its list of approved color additives after finding several had caused "serious adverse effects."

Now, more companies say they are replacing artificial dyes with colors made from fruits, vegetables and spices, which are widely considered "natural," although the FDA doesn't classify them that way. But these present more challenges than artificial dyes.

In addition to costing more, colors from fruits and vegetables can be sensitive to heat and acidity. And since they're used in higher doses to achieve boldness, tweaks to other parts of recipes may be needed. Such adjustments can be tricky for companies that manufacture on massive scales.

Still, companies want to court people like Heather Thalwitzer, a 31-year-old homemaker in Melbourne, Fla. Thalwitzer avoids artificial colors because she wants her 6-year-old son to eat quality food and she said red dye has been linked to "mania."

She has tried alternatives like naturally colored sprinkles from Whole Foods, which her husband thinks taste like fish. But she can get along without such products. One year, she made cupcakes topped with a single blueberry for her son's birthday.

There are times when Thalwitzer makes exceptions, such as when her son is at a friend's party.

"I'll let him have the birthday cake," she said. "But I'll cringe."

Part of the challenge with colors from natural sources is that the range of hues has been limited. Blues, for instance, weren't widely available the U.S. until 2013. That's when the FDA approved a petition by candy maker Mars Inc. to use spirulina extract as coloring in gum and candy.

The algae can now also be used in ice creams, drink mixes and other products.

"That was a big thing for us," said Stefan Hake, CEO of the U.S. division of natural color maker GNT.

At the company's office in Tarrytown, N.Y., Hake demonstrated how to get blue from spirulina by pouring a liquefied version of it through a coffee filter to isolate the right color components.

The approval of spirulina extract also opened up the world of greens, which can be made by mixing blue and yellow. It turns out plants like spinach brown in heat and aren't ideal for coloring.

Getting approval for a new color source can take years, but it's one way companies can fill out their palette of natural hues.

In coming weeks, an industry group plans to submit a petition to use the carthamus in safflower for yellow, according to color maker Sensient Technologies.

"It's just one more that might be another crayon in the crayon box," said Steve Morris, Sensient's general manager of food colors for North America.

Sensient also developed a "deodorizing process" to remove flavors from ingredients. That allowed it to introduce an orange for beverages made from paprika.

Morris declined to detail the company's process. But since the ingredient is not "fundamentally changing the form," he said the ingredients are still within FDA guidelines of permissible color sources.

Sensient said three-quarters of its new projects for clients in North America involve natural colors. Globally, its sales of colors - natural and synthetic - comes to about $300 million.

There are seven synthetic colors approved for broad use in foods. But these dyes can be mixed to create a wide range of colors. The colors are made by synthesizing raw materials from petroleum, according to the FDA.

Synthetic colors still dominate in the U.S., but some cite a study linking them to hyperactivity in children in calling for them to be phased out. Lisa Lefferts at the Center for Science in the Public Interest also says artificial colors can be used in deceptive ways.

"They mask the absence of ingredients," she said.