U.S. law attempts to balance the benefit to consumers of unbridled competition with the right of innovators not to have their creative efforts stolen. Accordingly, there's no guarantee your product's look will be protected. In trade dress cases, it's up to the originator of the product to prove that its design is inherently distinctive, so customers think of your company when they see that design. The more distinctive the shapes, colors and designs, the more defensible the trade dress claim. You also have to show that the design elements in question aren't part of the product's function. Finally, you have to prove customers are likely to be confused about where the product came from.
In many situations, the likelihood of customer confusion decides the case. Makers of generic products are able to get away with knockoffs because they state clearly on the label that this product should be "compared to" the name-brand version. Courts have ruled that when the products are displayed side by side with similar shapes and colors on the packages and the lower-priced generic invites consumers to compare their product to a given brand-name product, consumers know they're looking at a cheap imitation and can choose accordingly.