Fluke is not the first one to use the color scheme they were just the first to apply for a patent on it.
Even if they were they didn't enforce it until now so it became the general color scheme of all multimeters not just Fluke multimeters.
https://www.google.com/search?q=multimeter&safe=on&client=fi...
Most of the ones that look like a Fluke are indeed a Fluke.
[1] http://www.amazon.co.uk/s/ref=nb_sb_noss_2?url=search-alias%...
In any case, I don't think I'm qualified to have an opinion in the case being discussed here. Both because my understanding of trademark law is just a very general sense of what it is supposed to stand for and because I have absolutely no notion of what are the multimeter brands out there and what their designs look like.
For what it's worth, all the multimeters I recall using at school were either all yellow or all black and I have the feeling they were always the same brand although I don't remember what that was (I studied in Europe)
In the case of the Macbooks vs Dell XPS ultrabooks you could argue that what makes people recognize a Macbook is as much the glowing white apple logo as the aluminum case so people are not being fooled when they see a Dell XPS into thinking it's a Macbook.
Similarly, people don't immediately assume that a sport car painted red is a Ferrari. But if they were to see a car with a Ferrari logo (regardless of its color) they would most certainly assume that it's a Ferrari.
In short, it's my understanding that trademarks are by definition somewhat subjective. However I believe that the intention is a good one, as it protects as much customers than brands.