It's likely midnight tonight/tomorrow when the traffic light list is revealed.
Whichever colour Turkey falls under initially (and it's about a 99% certainty it will be amber at best) is what the rules and regs will be for the final.
By all accounts it's going to be reviewed every 2 weeks once travel goes live again from 17th, so if Turkey is amber tonight/tomorrow, it will be amber until at least Monday 31st May.
I think there is more chance of me waking up with Taylor Swift than the government giving travelling fans an exemption from any rules, even those on official club trips, so I wouldn't pin any hopes on a 'travel bubble' being authorised.
If Greece were to get green status and anyone thinking of holidaying there and travelling over to Istanbul for the final and back again, bear in mind you'd still be subject to amber restrictions on return to the UK. The only way round this would be to spend the 10 days after Istanbul in another green country (be that Greece or elsewhere)