My view is that you should use MathJax markup for things which are math (notation), and that is all. If that means you can't use color in the body of a post, that's okay.
For various reasons, we care about semantic markup here, which basically means using the markup commands to indicate what type of content something is, not to make it look the way you want it. When you use the MathJax delimiters, you are implicitly telling the system "this is math". It might get treated specially by search engines, summarizers, screen readers, and so on. So you shouldn't indicate that something is math when it isn't.
If you want to emphasize a key point of a post, we already have markup for that: either
_italic_, depending on whether you want the emphasized text to stick out at first glance or just to be stressed without disrupting the normal flow of reading. I'm not denying that there can be good ways to use color in text, but I think we can get by without it.
On the other hand, math notation is more limited, so it's often useful to have more ways of marking up the notation than just bold and italic; for example, to distinguish between different instances of the same variable, or to show significant vs. insignificant figures, or to track part of an expression through a computation. Color is useful in these cases. (Not to mention, bold and italic already have standard meanings in most mathematical notation.) I think the reasons we don't need color in text don't carry over so well to math, so I wouldn't support removing the ability to use color in MathJax.