private static function _get_luminance( $hex ) {
		$red   = hexdec( substr( $hex, 0, 2 ) );
		$green = hexdec( substr( $hex, 2, 2 ) );
		$blue  = hexdec( substr( $hex, 4, 2 ) );
		$max_rgb = max( $red, $green, $blue );
		$min_rgb = min( $red, $green, $blue );

		return ( $max_rgb + $min_rgb ) / 2 / 255 * 100;
	}