That's sometimes an issue that is obscured in certain student text books but which is of great practical importance.
The judgment in Case C-257/06 concerned the obligation imposed by Italian law on the importer of cosmetic products to provide the requisite national authorities with certain information on the products and their manufacturers.
The Court recalled that Directive 76/768 provided exhaustively for the harmonization of national rules on the packaging and labeling of cosmetic products (see, inter alia, Case C-77/97 Unilever, paragraph 24; Case C-220/98 Estée Lauder, paragraph 23, and Case C-99/01 Linhart and Biffl, paragraph 17). Consequently, when the question referred concerns harmonization at the EC level, the national measures relating thereto must be assessed in the light of the provisions of that harmonizing measure and not those of the EC Treaty (see, Case C-150/88 Eau de Cologne & Parfümerie-Fabrik 4711, paragraph 28; Case C-37/92 Vanacker and Lesage, paragraph 9, and Case C-324/99 DaimlerChrysler, paragraph 32).
Oh, and by the way, the Court held that the Italian obligations were compatible with Directive 76/768.