Look at ReplaceColor for details.

function ImReplaceColor(var Image: TImageData; 
                        X: LongInt; 
                        Y: LongInt; 
                        Width: LongInt; 
                        Height: LongInt; 
                        OldPixel: Pointer; 
                        NewPixel: Pointer): Boolean; cdecl;

ImagingExport (at line 143)