[Hong Kong] Wagtail ID

Yes, it is an Eastern Yellow Wagtail, subspecies M. t. taivana: note the yellow supercilium, yellow throat and dark ear-coverts (noticeably darker than crown); leg color is black in all Yellow Wagtails and hence not diagnostic.

According to the IOC World Bird List there are just two species, yes: Western Yellow Wagtail (Motacilla flava) and Eastern Yellow Wagtail (Motacilla tschutschensis). There might always be taxonomic changes in the futures (as there have been in the past)…

Both have a number of subspecies:

Motacilla flava:
M. f. flavissima - British Yellow or Yellow Wagtail
M. f. lutea - Yellow-headed, Kirgiz Steppe or Yellow-backed Yellow Wagtail
M. f. flava - European or Blue-headed Yellow Wagtail
M. f. beema - Syke's, Blue-headed or White-eared Yellow Wagtail
M. f. iberiae - Spanish or Iberian Yellow Wagtail
M. f. cinereocapilla - Ashy-headed or Italian Yellow Wagtail
M. f. pygmaea - Egyption Yellow Wagtail
M. f. leucocephala - White-headed Yellow Wagtail
M. f. feldegg - Black-headed Yellow Wagtail
M. f. thunbergia - Grey-headed, Dark-headed or Northern Yellow Wagtail

Motacilla tschutschensis:
M. t. plexa - East Siberian Yellow Wagtail
M. t. tschutschensis - Alaskan or American Yellow Wagtail
M. t. macronyx - Manchurian or Ussuri Yellow Wagtail
M. t. taivana - Green-headed or Kurile Yellow Wagtail

The English names are synonyms. The above list omits additional forms, like melanogrisea, zaissanensis, angarensis, simillima and some others, like hybrids. It's complex.