The MathJax c2 \color macro takes two arguments, the color and the math to be colored, where the v3 version follows the LaTeX approach, where it has one argument (the color) and switches the color for the rest of the expression.

The expression $\color{red}{Red}\ Black$ is rendered:

 In v2 In v3 $\color{red}{Red}\ Black$ $\require{color}$ $\color{red}{Red}\ Black$

To get the same effect in v3, use ${\color{red} Red}\ Black$ instead:

${\color{red} Red}\ Black$

Or you can use the configuration

<script>
MathJax = {
tex: {

before the script that loads MathJax in order to retain the original v2 \color macro.