1. Find the .AdiumMessageStyle you want to modify
2. Right click --> Show Package Contents (I can't remember if you can do this in Tiger without a plugin) --> Contents folder --> Resources folder
You'll have to figure out which .css file the text color for outgoing message display is configured, then you'll be able to modify it. Might want to find a free hexcolor picker to help you with the color codes.
Make sure you make a backup copy of the .AdiumMessageStyle just in case