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 v2In 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: {
    autoload: {
      color: [],            // don't autoload the color extension
      colorv2: ['color']    // autoload v2 color instead
    }
  }
};
</script>

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