Replaces pixels with OldPixel in the given rectangle by NewPixel.

function ReplaceColor(var Image: TImageData; 
                      X: LongInt; 
                      Y: LongInt; 
                      Width: LongInt; 
                      Height: LongInt; 
                      OldColor: Pointer; 
                      NewColor: Pointer): Boolean;

Imaging (at line 224)

OldPixel and NewPixel should point to the pixels in the same format as the given image is in.