If you are willing to explore the world of the Visio ShapeSheet, here is a simple little tip that will save you time in creating diagrams. I will show you how you can have your shape text color automatically change so that it is readable against the shape's fill color. As you change the shape fill color, the text will automatically switch to black or white to maximize legibility.
The full steps are shown in the video below
Visio Tip: Automatic Text Color from Saveen Reddy on Vimeo.
The fundamentals involve using the IF and LUM functions in the Visio formulas.
Simply set the Character Color (see the screenshot below) cell to this formula
=GUARD( IF ( LUM( FillForegnd ) >140, 0, 1) )
And your text color will automatically change as needed.