Suppose you want each depth level in your forum selector to have a different color, to stand out. If you're using the non-level-based selector (make sure Admin -> Settings -> Switches -> Level-based forum selector is off), you can do so. Go to Admin Panel -> Forums -> Forum Settings -> "Forum selector options HTML:" and change the value to
<option value="{CATID}" class="selectcolor{CATINDENT}">{CATINDENT} {CATNAME}</option>

Then go to Admin -> Themes -> Manage Templates -> Stylesheet and add this at the bottom:

option.selectcolor { background-color: green; }
option.selectcolor- { background-color: blue; }
option.selectcolor-- { background-color: red; }

Continue adding lines for as many levels of indentation as you have.

Note: If you use a different indentation character than the hyphen, you will have to replace the hyphens with your character in the stylesheet.