On 6 September 2012, the United States Court of Appeals for the Second Circuit concluded that fashion designer Christian Louboutin's signature red shoe sole can act as a trade mark.

This latest judgment partially overturned the 2011 ruling against Yves Saint Laurent ("YSL"), which had been sued by Louboutin when attempting to introduce a red shoe with a red sole onto the market. On 6 September 2012, US Circuit Judge Jose Cabranes concluded that “the district court’s conclusion that a single colour can never serve as a trademark in the fashion industry was based on an incorrect understanding of the doctrine of aesthetic functionality" and ultimately ruled that Louboutin's red sole trade mark is a valid and enforceable trade mark.

The red sole was registered by Louboutin as a trade mark in America in 2008, having been used since 1992. The Appeal Court accepted that the red sole has now developed "secondary meaning" in the public eye and that members of the public associate red soles with the Louboutin brand.

However, the decision also stressed that Louboutin's trade mark was limited to shoes with a red coloured sole and a contrasting coloured upper part of the shoe (for example, a red sole but a green upper part of the shoe). To grant anything further would effectively distort competition by allowing Louboutin a monopoly over the colour red.

Ultimately, the Appeal Court suggested that if YSL wished to create a monochrome red shoe with a red sole, this would not be prevented and would not infringe the Louboutin trade mark.

Perhaps this a victory all round; YSL can continue making monochrome red shoes, Louboutin can protect their trade mark in the future and the fashion industry knows that (in America at least) single colours can act as a trade mark.