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.
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.