(FORTUNE Magazine) – You're at a party and notice a bowl of cookies, each a different color. You're a bit peckish, so you reach in and grab one. It's purple--not just any purple, but that distinctly artificial purple used in candies, sodas, and bubble gum. And if there's one thing you've learned in a lifetime of interacting with such products, it's that this particular shade of purple signifies grape. Secure in this knowledge, you bring the cookie to your mouth, take a bite, and are surprised to find that it tastes like...vanilla.

That's a snapshot of the very odd eating experience provided by Keebler Rainbow Vanilla Wafers, surely one of the year's oddest product launches. The cookies, which first hit grocery shelves a few months ago, look just like conventional vanilla wafers--flat on the bottom, rounded on top, sort of like a miniature pitcher's mound--except that they come in a range of colors not usually associated with vanilla: purple, green, a watermelonish red, and a disturbingly Cheez Doodles-esque orange. It all makes for an extremely counterintuitive snack--even if you know what the wafers are supposed to taste like, it's hard to turn off the part of your brain that's been trained to equate specific colors with specific tastes.

Carolyn Burns, Keebler's marketing director for cookies, agrees that a purple or green vanilla wafer doesn't sound like the most obvious candidate for a new product. "In fact, our original plan was to make them fruit-flavored, much like a Trix or Froot Loops execution," she says. "We were even thinking about fortifying them with vitamin C. But our testing found that that's not what kids wanted--they just wanted a plain vanilla wafer in a variety of colors." Despite the extent to which flavors have become color-coded in many consumers' minds, Burns says Keebler has received no negative feedback on this point. "And frankly," she acknowledges, "I'm surprised."

The very concept of multicolored cookies is either groundbreaking or perplexing, depending on one's perspective. Although Keebler has gradually been building equity in its Rainbow brand extensions over the years, first with Rainbow Chips Deluxe and then with Rainbow Cookie Sticks, the rainbow element in those products is provided by multicolored candy add-ins, not by coloring the cookies themselves. Even Burns concedes that coloring those cookies "would not be visually appealing." So why color the traditionally golden-brown vanilla wafers? "Because the demographics marry up perfectly," Burns explains. "The Rainbow products skew toward young children, and so do conventional vanilla wafers, so it made all the sense in the world." While the finished product didn't end up fruit-flavored, as Keebler had envisioned, it's hard to argue with the initial sales figures, which show the colored wafers outselling Keebler's conventional brand 53% to 47%.

While such numbers are encouraging, it's important to maintain a sense of perspective here. Keebler didn't even get into the vanilla wafer market until the late 1960s, by which time the category was dominated by Nabisco, which first introduced the product in 1898 and, in a textbook example of vernacular branding, renamed the cookies Nilla in 1967. Nilla, a brand name that has since become virtually synonymous with "vanilla wafer," currently outsells Keebler's conventional wafers by about three to one, although Burns sees this as a potential advantage for attracting Nilla customers to the new Rainbow product. "That's the beauty of line extending when you're No. 2, as we are," she says. "Even if we draw consumers at equal rates across the entire vanilla wafer business, that means a disproportionate share of it will come from them, not from us."

Of course, that will all go up in smoke if Nabisco comes up with a multicolored wafer. Is such a product on the drawing board?

Nabisco didn't return calls seeking comment, but Burns says "it wouldn't surprise me" if a copycat wafer were in the works. Meanwhile, the preliminary success of Rainbow Vanilla Wafers raises an obvious question: Is Keebler considering colored versions of any of its other cookies? "It's definitely in the back of my mind," says Burns, "but it would have to be the right product." For what it's worth, here's one consumer's advice: Leave the chocolate cookies brown.

--Paul Lukas