Color in MathJax

If you are testing your site locally using a file:// URL, then Firefox's same-origin policy probably is forcing MathJax to use image fonts rather than web fonts. See the MathJax FAQ entry on this, in particular the section on Firefox local @font-face. One solution is to install the MathJax fonts locally on your computer where you are doing the testing. The fonts can be obtained from the MathJax GitHub repository (click on a font and then on the "Raw" button in the upper right).

Alternatively, you can simply ignore the black color during debugging, as it will work properly once transfered to a live website.

MathJax supports the \color macro, but it works differently from LaTeX. In LaTeX, \color works like plain TeX commands such as \bf, but MathJax takes it as a LaTeX command like \textbf.

\[ \color{red} The whole sentence is red in LaTeX,
   but only the initial T is colored in MathJax.  \]