change ColorScheme without any results

I am evaluating Imaging/Barcode/PDF Tools EVAL 14.5 for a Delphi VCL project. I would set
all twain parameters into the code (Without a Twain Dialog).
 
If i set the ColorScheme parameter from TWPT_BW to TWPT_GRAY i can see no result. 

- if i change the "HorzResolution" or other settings i can see the results
- if i change from "AcquireMulti" to "Acquire" i can see the change from TWPT_BW to TWPT_GRAY
- if i change the parameter about the twain dialog i can see the correct result

Here is the code...

     LEADTwain1.SelectSource();
     SetFocus();

     FillMemory ( @twProps, sizeof(LTWAINPROPERTIES), 0 );

     nRet:= LEADTwain1.GetProperties ( @twProps, LTWAIN_PROPERTIES_GETCURRENT );

     if ( nRet <> SUCCESS ) then
        begin
        Exit;
        end;

     twProps.DataTransfer.nTransferMode:= TWSX_FILE;
     StrCopy ( twProps.DataTransfer.szFileName, 'd:\temp\Twain.tif' );
     twProps.DataTransfer.nScanFileFormat    := TWFF_TIFFMULTI;
     twProps.DataTransfer.nBufMemCompression := TWCP_NONE;
     twProps.ImageRes.fHorzResolution        := 300;
     twProps.ImageRes.fVertResolution        := 300;
     twProps.ImageEff.nColorScheme           := TWPT_GRAY;
     twProps.nDuplexScanning                 := 2;

     LEADTwain1.EnableSetPropertiesEvent:= True;
     nRet:= LEADTwain1.SetProperties ( @twProps, LTWAIN_PROPERTIES_SET );
     if ( nRet <> SUCCESS ) then
        begin
        Exit;
        end;

    LEADTwain1.EnableAcquireMultiEvent:=True;
    LEADTwain1.EnableAcquirePageEvent:=True;
    nRet:= LEADTwain1.AcquireMulti ( 'd:\temp\test.tif', LTWAIN_SHOW_NONE, LTWAIN_BUFFER_MODE, FILE_TIF, 1, TRUE, 0, TRUE );

Thanks for your help.