
If you want to play with it then just edit the hex values in the fretsonfire config file (fretsonfire.ini) which is in the same folder as the .exe folder (backup the original just incase, saves having to re-extract it, etc)
you can find colour (not sodding 'color', wtf is that about?) hex values here - http://www.w3schools.com/html/html_colors.asp

The example above uses the following values:
base_color = #F00000
background_color = #FFFFFF
selected_color = #FF9900
'base_color' is the colour that unselected text is and the other 2 are pretty self explanatory.
Enjoy