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;