In the Data Graphic, when using Color by Value, the fill color will correctly change based on linked data. But the Text Color is always black regardless of what color you choose.
I tested in Visio 2007, and text color changed correctly according to the width of the shape.
How did you set text color by values?
I have the exact same issue in my Data Graphics. Even if I set the text colour to be white in the settings it is black on black.
In the end I select the box and manually make the text white.