Found this on the Zumo forum. Gave it a try and it works great! If your Zumo supports map themes you know most of the optional themes are better than Garmin's default, for example our favorite back roads are easier to see. With this theme editor you can customize a theme to your liking (now unpaved roads are dirt colored and secondary roads stand out
). Be sure to rename the theme so you still have the original in case you mess up. Also, I downloaded a theme to edit on my PC rather than on the unit, just to be safe.
http://www.javawa.nl/zumocolors.html