<!doctype html>
<html>
  <head>
    <title>fractions color</title>
    <meta charset="utf-8"/>
  </head>
  <body>

    <math>
      <mfrac>
        <mtext><span style="display: inline-block; width: 3em; height: 1em; background: red;"></span></mtext>
        <mtext><span style="display: inline-block; width: 5em; height: 1em; background: green;"></span></mtext>
</mrow>
</mfrac>
</math>

</body>
</html>