It depends upon what the personal preferences of the person who connected the old one used...??
If it was me I would have used
Brown perm Live.
Black (Sleeved brown) switched live.
Grey (Sleeved blue) neutral.
But I have seen all sorts of combinations used over the years..
Basic rule is never assume anything until you have proved what you think you know is actually correct, before reconnecting the power!!
You should have noted what colours were used for each function on the old fan before disconnecting it..
Now you either need some test gear the verify which wire is which..
Or if you can access the other end of the cable, e.g. fan control switch / or isolator, you may be able to get some ideas..
But it may also go to a junction box?
