That definition of a grey day is grey and susceptible to roster abuse.
A more definitive wording would be:
A Grey Day is neither a rostered Day Off nor a rostered Duty Day. The Company may allocate duty on a Grey Day and if mutually acceptable to the Crewmember, will make him/herself available for such duty.
It has been practice by some Companies to roster a group of "grey days" and then fill them in with duty as and when required.
This is done for two reasons:
1. There is no duty reflected on a pilot's roster as in Reserve or Standby. Therefore avoiding any possible CAO 48 limitations.
2. It effectively provides the Company with an unlimited "Clayton's" standby
period.
However, this type of rostering may also diminish the pilot's ability to secure adequate rest before flight and therefore, if this occurs regularly, can impact on fatigue management for the overall roster period.
I believe CASA are now scrutinising rosters more thoroughly these days during their audits.