Very nice trademark law answer. I'm actually working on my Trademark Law outline in preparation for my upcoming exam in two weeks. Don't forget the huge likelihood of confusion caused by Forever21's red soles. And the secondary meaning that Louboutin's red soles have acquired as increasing numbers of people on a global scale instantly recognize the red soles as being distinctive and attributable to Christian Louboutin.
However, I will point out that a copyright is not the same as a trademark. And copyright law has different statutes and regulations than trademark law. With that said, Christian Louboutin has a trademark in one shade of red on the soles of shoes, but Louboutin does NOT have a copyright in the red sole.
I feel kind of bad for Cesare Paciotti, who was the first shoe designer to use red soles regularly. He's probably wondering why his luxury shoes aren't nearly as popular as Louboutins. I also love Paciotti's shoes and his trademark silver dagger engraved on the red soles.
When I said it has to meet certain requirements that included secondary meaning and likelihood of confusion. If you look up Louboutin's trademark registration on the USPTO, it only says RED, not a particular shade of red. If another designer used another shade of red, it would have to be determined by a court whether or not the two shades were similar and whether consumers would likely be confused as to the source of the product. At this point, I do not even think his color has a number on the Pantone scale, but I may be wrong.
Paciotti's red is not as bright as Louboutin's, and it is distinctive because of the dagger, and also most of the shoes that I have seen from him only have the red in the arch of the sole, not on the ball of the shoe. If the red soles aren't working for Paciotti, maybe it's time for him to change things up to make his designs more popular, that is what competition is all about: survival of the fittest.
Copyright does not apply here in any way shape or form. You can only copyright sound recordings, dramatic works such as dances, movies and other audiovisual works, pictorial, graphic or sculptural works, architectural works, musical and literary works. Color falls solely under trademark law and trade dress, not copyright.
Although Louboutin may want to try to obtain a copyright for his actual designs if Congress passes the Design Piracy Protection Act, to prevent companies such as Forever, Madden, Oh Deer! from blatantly copying. Although even though I am a diehard fashionista, I don't know if I necessarily agree with the Act in its entirety.