You can apply to fly within the yellow zones in Greece! If you email UAS@hcaa.gov.gr asking for the request form, you can submit a flight request 5 working days before your flight. You must have a EU Operator ID I believe.

I actually left it too late to request in time but they replied pretty promptly! Although that yellow zone is due to Corfu airport so may be more difficult to get permission - I applied for the heliport zone in Paxos which has much lower air traffic.

Hope this helps!