Good question
The key feature to look for is whether the under tail coverts (UTC) are dark with pale edges (Verditer) or all-pale (Pale Blue). Your picture shows part of the UTCs and these look to me to be completely pale.
Pale Blue is supposed to be longer-billed (hard to tell at this angle) and have a greyer belly (looks like it)
I'd be interested in others' opinions
Cheers
Mike
Mike KilburnVice Chairman, HKBWSChairman, Conservation Committee