Manhattan federal judge Victor Marrero had all kinds of fun in the 32-page opinion in which he toppled one of fashion's most recognizable trademarks, the shiny red sole of Christian Louboutin's pricey high heels. The judge quoted J. Lo's 2009 pop song Louboutins, whimsically inserted details of the case into a Walt Whitman poem, and spun out a fantasy in which fashion designers rushed to the U.S. Patent and Trademark Office to lay claim to various colors of the rainbow. In the end, as Reuters reported Wednesday, the judge decided not only that Louboutin can't enjoin Yves Saint Laurent from selling shoes with a red sole, but that the Patent and Trademark Office shouldn't have awarded Louboutin a trademark on red soles at all.

That decision turned on a question that has never before been asked in federal court. Is a red-soled shoe more like pink fiberglass insulation or a green tractor?

First, a little bit of fashion history. In about 1992, the shoe designer Louboutin painted the bottom of a pair of shoes with red nail polish and launched a phenomenon. As Judge Marrero wrote, "He departed from longstanding conventions and norms of his industry, transforming the staid black or beige bottom of a shoe into a red brand with worldwide recognition at the high end of the women's wear, a product visually so eccentric and striking that it is easily perceived and remembered." No dummy, Louboutin went to the PTO to make sure no other shoe designer could take advantage of his red-bottomed brand recognition. In 2008 he was awarded a trademark, and Louboutin lawyers began enforcing the mark to block knock-offs.

In 2011, Louboutin informed Yves Saint Laurent that four lines of shoes from YSL's 2011 cruise collection infringed the red-sole trademark. The YSL shoes were entirely red, not just red-bottomed; Saint Laurent's lawyer, David Bernstein of Debevoise & Plimpton, told me that Yves Saint Laurent himself created the concept of a monochromatic shoe back in the 1970s, and the YSL line has occasionally introduced one-color shoes ever since. (Bernstein added that YSL has no desire for its shoes to be confused with Louboutins and would, in fact, be insulted by any such confusion.) YSL stood firm in its red high heels even when Louboutin's lawyers at McCarter & English filed a Manhattan federal court Lanham Act suit, seeking to enjoin the YSL red shoes.

Enter Judge Marrero, who seems to have been delighted to preside over a case that presented a novel legal question-can a fashion designer trademark a single color?-in the rarified context of high-end high heels. The issue, as Judge Marrero framed it, was whether Louboutin's red soles serve an essential function, "meaning that the color is essential to the use or purpose of the product," or serve only to distinguish Louboutin shoes from competitors' pumps and strappy sandals. If Louboutin uses red just so that everyone who sees a flash of lacquered color when a woman walks by will know she's wearing Louboutins, then the red sole is trademarkable. If the red has "expressive, ornamental, and aesthetic purposes," Judge Marrero wrote, it can't be trademarked.

The judge pointed to Owens-Corning's pink fiberglass insulation as an example of a product whose color merits a trademark. There's no reason for the insulation to be pink, Marrero explained, except to identify it as an Owens-Corning product. (The judge didn't cite Tiffany's trademarked robin-egg blue, but the same reasoning applies to its packaging as to Owens-Corning fiberglass.)

John Deere, on the other hand, couldn't get a trademark on the color green for its farm equipment because an Iowa court held the color to be functional. (As Judge Marrero quotes the ruling, "[farmers] prefer to match their loaders to their tractor.") Similarly, Brunswick couldn't trademark the color black for its speedboat engines because black is functional. (It matches a lot of boat colors.)

Alas for Louboutin, Judge Marrero found red soles to be akin to green tractors and black boat engines. In fashion, the judge concluded, color is an aesthetic decision that serves an essential function. He quoted Louboutin himself, who said he chose red because it's "sexy," "engaging," and "attracts men to the women who wear my shoes."

"The [bottom] of a shoe is, almost literally, a pedestrian thing," Marrero wrote. "Yet, coated in a bright and unexpected color, the outsole becomes decorating, an object of beauty. To attract, to reference, to stand out, to blend in, to beautify, to endow with sex appeal-all comprise nontrademark functions of color in fashion."

Louboutin's lawyer, Harley Lewin, told Reuters that Marrero made a mistake. "It appears to us that while acknowledging the trademark status and renown of Louboutin's famous Red Sole trademark, he has unilaterally decided that a single color cannot be a trademark in the fashion industry," Lewin said. "We disagree and are evaluating the alternatives available."

But YSL counsel Bernstein of Debevoise told me the judge got it exactly right. "The function of fashion is to communicate something," he said. "These designers view themselves as artists." (Bernstein also said he had a blast working on the case: "Cases like this are why it's so much fun to be a lawyer in this area.")

I asked Bernstein whether Louboutin could have trademarked a red design-polka dots, say, or a thick red stripe-on the bottom of shoes, instead of simply a color. "If he came up with something truly unique and creative he might be able to win a trademark," Bernstein said. "But that's not what he did."

(Reporting by Alison Frankel)