This is one of the ways you change the font in Vista. You can try this method to see if it will work in Windows 7:
Right click Desktop, then Personalize, and select Window Color and Appearance. Click on the Advanced button and on the next screen, under 'Item' you will see a drop-down menu of approximately 20 items. Each of those items controls a number of different size/color/font/screen settings, one of which will be the font color for the Taskbar. You'll have to experiment with them and try each one to see which item controls what, because I don't know which will control the Taskbar. The only bad news, is that if it is the last one, Window, then the system will not allow you to change the font color for that setting.
Good luck, and let us know if you had success.