Go to the menu panel on the left side of Encore and then click above the menue panel in the very top of the Encore window to select MENU, that's the fourth selection over to the right from FILE.
If you uncheck the automatic menu assignment you are then free to move to the group of menu highlights below to modify the color and saturation associated with the buttons of your menu. Each color and the opacity of same can be modified to give you the effect you desire, in terms of changing the color of the button text or shape or both.
I would highly suggest that you get the Lynda.com training series on Encore CS3 by Chad Perkins. It is a well documented overview of Encore and it will be well worth the one time investment if you only want to purchase a limited number of downloads from Lynda.com. In fact, I have found them incredibly inexpensive for what you are exposed to in a very short time.