The tail pattern is useful here - you can clearly see on this bird that the tail and uppertail coverts are black, with a small white patch on the base of the outer tail feathers. This tail pattern can only be found on Red-throated and Red-breasted (treated as the same species until recently).
Red-breasted should have browner uppertail coverts and usually a paler base to the bill, so I agree this is Red-throated (which is also more likely than Red-breasted, which is rare in HK).
