Lets imagine the outcome of two theoretically unrelated possibilities:

1) For safety reasons someone (who?) has purchased an OSHA regulation which now requires that all electricians use high contrast yellow multimeters, yellow indicating their, uh, safety-ness or something.

2) Fluke has paid the .gov to be the only legal yellow multimeter supplier.

I know #2 is true. I suspect but have no evidence #1 is true.

What a pointless thing to enter into your widely connected information appliance. If you think it is true and has some merit as an idea, spend the couple minutes to actually inform us, alternatively just don't bother telling us what you think might be true.

Is it then impossible to sell a product because it would always interfere with someone else's color?

BTW, I agree with other EEs, the Sparkfun meter is not "really great high-quality" as they claim. It will suffice for hobby use, but Fluke's markup is based on having additional quality, not brand recognition.

1) You can't just apply to protect an arbitrary color. You need to show secondary meaning (i.e. the market associates the color in a certain context with your brand). It is unlikely for that to happen with more than one brand in a particular space.

2) The color can't be functional and can't affect the quality or value of the product. So Apple can't trademark aluminum-colored laptops, nor can you trademark yellow or orange traffic cones.

3) With respect to running out of colors, that issue is addressed on pages 168-169. The gist of the argument is that if people "use up" the high-contrast color schemes, than not being able to use some mark will put competitors at a functional disadvantage, which will invoke the functionality bar mentioned in (2), invalidating the other color marks.