Looks good for Citrine to me. It has a very broad supercilium, so if a Yellow Wagtail it can only be taivana (of the taxa occurring in HK) which would have solid and dark ear coverts. This bird has classic Citrine pale centred ear coverts. Some Citrines do not have a complete pale border to the ear-coverts, although this does look complete in the second photo. Pale forehead is also a good Citrine and anti Yellow feature. Bill also looks long which again favours Citrine.
