This brings up a good point. At what point is it not ok to trademark a color? What if a company decides they want to trademark blue, green, red, orange, etc etc to block competition. They could say 'look, fluke got one for their yellow' and thats a precedent they can use.
I think if someone truely wanted to fight it, they could get that color trademark stuck down. Not saying it would be easy, or cheap, but I think it would be possible.