I would agree that you can trademark an appearance but yellow is not any kind of identifiable color. This in my mind would make a trademark too vague to hold up

I would agree that you can trademark an appearance but yellow is not any kind of identifiable color. This in my mind would make a trademark too vague to hold up in court. But my mind != stupid US government agency. I wonder if you can trademark objects with colors outside human vision. "I'm sorry sir your multimeters glow in the ultraviolet and we own that trademark." It is also unreasonable that there is no option once seized to have it ship to a country other than the origin. After all it may very well be legal in Canada or Poland or somewhere.