This is a female Flowerpecker.
I think it is a female Scarlet-backed Flowerpecker but it is difficult to tell from this picture whether the bird has the scarlet rump distinctive of that species. Looking closely at the photo, I think I can see a suggestion of red close to the tail but it is very hard to tell (do you have other photos which show the rump).
It also looks greyer above (especially around the face) than I would expect for female Fire-breasted, which is the other common species in HK but I am not entirely sure about the ID for this individual from these photos. Call and habitat can be useful clues for an ID of these species.
Congratulations on the photo by the way. Flowerpeckers are usually very active at the tops of trees and very rarely give such good views as this (which is one reason that they are often so hard to ID!)
