protected function writeColor(XMLWriter $objWriter, Color $color, $alpha = null)
{
if (is_null($alpha)) {
$alpha = $color->getAlpha();
}
// a:srgbClr
$objWriter->startElement('a:srgbClr');
$objWriter->writeAttribute('val', $color->getRGB());
// a:alpha
$objWriter->startElement('a:alpha');
$objWriter->writeAttribute('val', $alpha * 1000);
$objWriter->endElement();
$objWriter->endElement();
}