Custom colors for collectd meta graphs not working with PHP 7

When using PHP 7, in custom collectd meta graphs, colour definitions are not parsed correctly in class CollectdColor and result in all the colors being black ('000000'). This seems to be caused by a change of parsing hex strings with PHP 7. Strings of hexadecimal numbers are no longer considered numeric as explained described in
http://php.net/manual/en/migration70.incompatible.php#migration70.incompatible.strings.hex

The attached patch uses the recommended parsing via filter_var.