BOOL InvertRgn( CRgn* pRgn );
pRgn
Identifies the region to be inverted. The coordinates for the region are specified in device units.
Inverts the colors in the region specified by pRgn. On monochrome displays, the function makes white pixels black, and black pixels white. On color displays, the inversion depends on how the colors are generated for the display.
TRUE if the function is successful; otherwise FALSE.
CRgn, ::InvertRgn