The font text color for a specific instance of the PI TreeView web part is not configurable. The color is based on the currently selected Site Theme. If desired, you can change the text colors in the active theme, but this will affect not only the TreeView web part but all other SharePoint web parts as well. Specifically, in SharePoint 2010, it appears that the color chosen for "Text/Background - Light 2" determines the color of text for elements displayed in the tree. I hope this is helpful.