The dark carpal bar (on the leading edge of the wing) indicates that this is a first summer bird. This would explain why the bird is not in full breeding plumage (pale forehead, dull bill). Other plumage aspects and structure seem right for this to be a Roseate Tern.
Terns usually do not breed in their first summer and rarely return to the breeding grounds at this time (most stay in the wintering areas), which is why this plumage is not usually seen in HK.
A very good set of photos of this interesting bird.
