Code Grey covers the situation where all the met criteria does not generate the forecast of fog neither real or the possibility thereof. If there is a remote chance of fog occurring based on aches in grannies bones or the ants are stirring or any other meteorological condition, then a cOde Grey is issued by BOM. (In the old days BOM used to issue forecasts with prob 5% or 10% fog to cover grannies bones but these days the criteria is that if the chance is less than I think 30% then fog is not mentioned).
The concept was introduced by QF and fuel policy dictates that fuel for an alternate must be carried from a pre- flight point of view when there is a code grey.
Not sure how many other airlines use it , if any.
QF uses it because lots of flights don't carry alternate fuel if the weather forecast doesn't demand it.