Well, in HK the situation is actually easy, because Paddyfield Pipit does not occur here. It is a resident bird that occurs in countries such as India, and in South-East Asia (Thailand, Malaysia, Vietnam,…). As it is a non-migrating bird, it is very unlikely to turn up in HK.
But Richard's Pipit and Paddyfield Pipit both occur in some parts of the above mentioned countries in winter (Richard's then migrates south).
The easiest way to separate both is by their call: 'chep' in Paddyfield and a loud, House Sparrow-like 'shreep' in Richard's.
Separating the two species on plumage is not easy (I have never seen Paddyfield Pipit). Paddyfield is a bit shorter-tailed, has less distinctive streaks on the head, and a dark loral stripe. But depending on the position of the bird on a picture, that loral stripe may be not always straightforward to judge.
The bird in your picture could, however, potentially be a Blyth's Pipit, although I think is has been seen only once in HK, in autumn (it winters in India).
The call of Blyth's is similar to that of Paddyfield, but apart from that it has a second call that resembles Richards' Pipit, but is a bit less harsh.
Blyth's is smaller in many respects: posture, shorter tail, shorter legs, shorter hind claw and shorter, finer bill. Different from what can be seen on your pictures.
The median coverts are another character that can be judged from your first picture. The dark pattern is pointed towards the edge and the shape of the dark area is triangular. In Blyth's it is less pointed (i.e. not so far to the edge) and triangular. As can been seen here:
http://www.avianweb.com/blythspipits.html.
I think most confusion is with smaller Richard's that is than considered being Blyth's. But your bird is clearly a typical Richard's.
Cheers,
Bart
