In the Thread Prefix Manager, edit the prefix, and in the "Title (Rich Text)" field, enter the html you want to add color. For example, for Tech you could enter:
Code:
<span style="background-color:green">Tech</span>
or you could use classes instead, and put css for th class in the additional.css template.
Is that what you meant, or do you want to change the backgroud of the entire thread based on the prefix?