Hi Mike,
What Windows XP does is changing the colors. XP can do this on a much better global level than we can in a VFP application. However, the idea would be the same. Basically, you need to loop through all your objects and change all color properties. To get the desired effect you need to work with HSL color codes instead of RGB (expand the GetColor dialog to see what I mean). To turn a color into gray you steadily reduce the saturation (text box in the middle) to 0.
The older Windows effect is easier to achieve. Basically it's an image where one pixel is black and one transparent.
--
Christof