Q: I'd like to brand and sell a dog toy that has a trademark. What changes (color, size, design) do I have to make ?

A: Questions of infringement are not straight forward (only patent infringement is addressed in this answer). One needs to research if there is something to infringe, such as a utility patent or a design patent. Then there needs to be a determination of infringement.

There is possible utility patent infringement. For utility patent infringement, there needs to be a preponderance of the evidence that the accused product infringes one or more claims of the patent either literally or under the doctrine of equivalents. Bayer AG v. Elan Pharmaceutical Research Corp., 212 F.3d 1241, 1247 (Fed. Cir. 2000).

There is possible design patent infringement. A determination of design patent infringement is done by the “ordinary observer test.” Under the ordinary observer test, an accused product infringes a design patent if in the eye of a hypothetical ordinary observer the two designs are substantially the same, such that the resemblance is such as to deceive such an observer, inducing him to purchase one supposing it to be the other. Curver Luxembourg v. Home Expressions Inc., 938 F.3d 1334, 1338 (Fed. Cir. 2019)

These are simple interpretations of the standards of law that need to be analyzed--and compared to the specific set of facts relevant to you. That said, don’t let the potential risk, or legal terms, prevent you from pursuing a potentially good idea (a full analysis will determine if it is a good idea). Consider hiring an intellectual property attorney to look over the specific facts of your situation—in an effort to determine potential risk, then you can hopefully go forward if everything looks good.

This answer includes generalizations and there are many caveats. The answer does not form an attorney client relationship.