I don't think you'll find a "tool". But, in code view:
<p>this text is normal <span style="color:red">this text is red</span> this text is normal</p>
<p>this text is normal <span style="background:red">this text is black on red background</span> this text is normal</p>
<p>this text is normal <span style="color:yellow;background:red">this text is yellow on red background</span> this text is normal</p>
No guarantees how this will display on monochrome devices though! And some colour devices ignore highlighting completely.