When I was in law school, one of the hot topics in the trademark world was whether "color per se" (color without more) could act as a trademark. At the time, there was a Circuit split regarding whether "mere color" could receive trademark protection. Some of the cases involved the color of fake sugar packets, the color of insulation, or the color of cleaning press pads. In 1995, it was the cleaning press pads case where the Supreme Court resolved the question about the trademarkability of color per se.
In that case, the Supreme Court ultimately ruled that, as long as the color had acquired distinctiveness (secondary meaning), then it may obtain protection as a trademark. In other words, if consumers came to understand that the color indicated the source of the product, then color is trademarkable.
Today, AstraZeneca is arguing that the color purple of its heartburn pill, Nexium, is a protected trademark. In so doing, AstraZeneca is able to block a generic drug maker from selling its generic version of Nexium. AstraZeneca notes that it heavily promoted Nexium as the "purple pill." It is this big advertising campaign which helps consumers to come to connect a purple pill with Nexium.
If you have read this blog before, you will know that having a trademark registration allows a trademark owner to block the importation of infringing goods. This is what I believe AstraZeneca did to the generic drug manufacturer trying to get its generic Nexium here. If I were to guess, I would say that the generic drug manufacturer will have to choose a different color for its generic Nexium before it could sell it here in the United States.