It's possible to change things like text color or font weight by including CSS inline styles in your source code. The source code button appears when a field supports HTML.
Click on the brackets to edit the source code:
<p style="color: red;">This is a sentence</p>
You could get more specific about the color by using a what's called a hex code. For example, use '#03a9f4' to turn the sentence a light blue:
<p style="color: #03a9f4;">This is a sentence</p>
For more information on how CSS styling works check out this article from the Mozilla Development Network.