Code:

procedure TForm1.Button1Click(Sender: TObject);

var

OldBkMode : integer;

begin

with Form1.Canvas do begin

   Brush.Color := clRed;

   FillRect(Rect(0, 0, 100, 100));

   Brush.Color := clBlue;

   TextOut(10, 20, 'Not Transparent!');

   OldBkMode := SetBkMode(Handle, TRANSPARENT);

   TextOut(10, 50, 'Transparent!');

   SetBkMode(Handle, OldBkMode);

end;

end;