If it's blueFrog or the Touch dialer from the WM6 kitchen, you could try...
Code:
[HKEY_LOCAL_MACHINE\Security\Phone\Skin\DIALER\PORTRAIT\Text]
"TextColor"=dword:00ffffff
[HKEY_LOCAL_MACHINE\Security\Phone\Skin\DIALER\LANDSCAPE\Text]
"TextColor"=dword:00ffffff
That's the default white, and all 0's should get you black. Should only affect the dialer screen and not the progress screen. Let me know if it works.