Trix? Check. Golden Grahams? Check. Reese’s Puffs? Check.

In January, General Mills announced that it would be reformulating these cereals with no artificial colors or flavors. One cereal was notably absent from the list, though.

It turns out, adding natural colors to marshmallows is a stickier wicket, meaning a makeover for Lucky Charms would be more of a challenge.

Removing the artificial ingredients while retaining the classic flavor of a bowl of Lucky Charms has sent the food scientists at General Mills back to the proverbial drawing board time and again. After all, how does one retain the vibrant hue of the blue crescent moon without Blue #2? Would the flavor of blueberry extract be too overpowering? How about drawing from plums, grapes, or even earthy beets?

And the moon is just one marshmallow type in the bowl. There are multi-colored rainbows, pink hearts, yellow hour glasses, and neon-green leprechaun hats, too.

“Each one brings a different challenge to the table for us,” says Kate Gallager, a cereal recipe developer at General Mills.

It has turned the quest to get Lucky Charms to look and taste right into an art form of its own.

On any given day at General Mills, six or seven people clock into work to actively pursue a recipe that works, Gallager says. They have an ever-changing pantry to work with, too, as each color requires screening new vegetables, fruits, and spices for a potential color match. Playing with concentrations of different juices, mixing them with marshmallow cream, and testing how they react to milk are all part of the job.

For each marshmallow conquered, the food scientists must then step back and consider the state of the entire bowl, paying keen attention to any small difference in taste. The subtlety of Lucky Charms—versus the loud, fruity flavors one would find in a bowl of Trix—makes the task of achieving vibrant colors with muted flavor all the more challenging.

General Mills hopes to introduce the new, all-natural Lucky Charms to market by the end of 2017. But Gallager says the company is prepared to tinker for as long as it takes to get the recipe right.