This is a first-winter Yellow Wagtail, subspecies macronyx.
Grey and Yellow Wagtails can be separated in a number of ways. One thing to remember is that first winter Yellow Wagtails in Hong Kong are mostly grey & white when they arrive and only gradually moult into the green and yellow breeding plumage.
Head pattern - Grey Wagtail always shows a clear, white, narrow supercilium (although some Yellow Wagtail, especially tschutschensis, can be similar)
Leg colour - black in Yellow Wagtail, pinkish in Grey Wagtail (note that the legs on this bird are stained by mud)
Undertail - Grey Wagtail has yellow undertail coverts throughout the year, many Yellow Wagtails are whitish
Upperparts - Grey Wagtail is always grey above, usually with obviously black wings. Any green on the upperparts indicates a Yellow Wagtail
Rump - Grey Wagtail has a yellowish rump, Yellow Wagtail is grey or green (not visible on this bird)
Tail - Grey Wagtail usually looks very long-tailed, Yellow is usually shorter-tailed
Wing-bars - In flight Grey Wagtail always shows a broad white wing bar formed from white bases to the primaries/secondaries, which Yellow Wagtail lacks. On the ground Yellow often has yellowish wingbars from the tips of the greater coverts.
Habitat - Grey Wagtail is usually in fast-flowing streams, Yellow Wagtail is usually in wet fields or fishpond bunds. This isn't 100% reliable, but is a useful guide.
I hope that helps.
