How to call variables in conditional formatting elements ?

I search how to call a variable holding color label in conditional formatting. This is working: <?if:ADVICE='CRITICAL'?><?attribute@:color;'Red'?><?end if?>
I tried:
<?xdoxslt:set_variable($_XDOCTX, 'DgBkColor', ’Red’)?>
...
<?if:ADVICE='CRITICAL'?><?attribute@:color;xdoxslt:get_variable($_XDOCTX, 'DgBkColor')?><?end if?>

But does not work...

The following does not work anymore:
<?xdoxslt:set_variable($_XDOCTX, 'DgBkColor', " ’Red’ " )?>

Regards