On these photographs, the blue on the throat and breast is much too extensive for Blue-throated (of any subspecies) but fits perfectly for Hainan Blue, which is a fairly common species in Hong Kong (and presumably nearby parts of Guangdong).
Juvenile Hainan Blue has a brownish plumage. The orange-brown feathers around the breast and throat of this bird are almost certainly the remnants of juvenile plumage, lost as the bird moults into an adult-like first-winter plumage (the older, browner feathers in the wings indicate that this is a bird hatched earlier this year). This is a possible pitfall with claiming Blue-throated in Hong Kong (or Guangzhou). Whilst there is a possiblity that some recent 'blue-throated' claims are in fact moulting Hainan Blue, at least some show a distinct orange breast and well-marked division between the blue and orange which would suggest otherwise.
I'm sure there is still more to learn about the ID of these species (especially females) and their pattern of occurrence in south China (are the recent birds all escapes?)
