As a general rule, in order to be protectable trade dress, a feature of a product, for example, the product's color, must be nonfunctional. That is, the feature must have been an arbitrary choice added to identify the source of the product (think of Owens Corning's pink insulation or T-Mobile's use of magenta).

Where a party seeks a utility patent to protect that very feature, it is making a public claim that the feature is not arbitrary at all, but instead has a functional purpose.  While not necessarily dispositive, such a public statement can doom claims for trade dress.  

In the case at hand, a company registered a patent to protect its use of a naturally pink metal in its product (thereby evidencing the functionality of the metal), and later tried to protect that color as trade dress.  While it seems that it could have easily chosen to eschew patent protection in favor of trade dress protection for the color, absent patent protection, competitors would be free to use the same metal so long as they undertook efforts to change its color.  As such, the company had to make a choice between eating and having its pink cake.  Moreover, even assuming the company didn't file a patent application or otherwise make public claims about the benefits of the feature, such information could still come to light.

Before relying on trade dress protection, companies are well advised to consider the origins of such features present in their products.