It doesn't look like a Tristram's, which I would expect to look more rufous around the body and darker around the face. It's not the best photo to show the face, underparts or tail but looks to be yellow underneath, with a reasonably pronounced face pattern. Like I said in the previous message, I think it's probably a female Black-faced.
I'm not sure where you mean by "NSE" - in most places in HK, Black-faced is the commonest bunting at this time of year. Tristram's is more or less confined to woodland/forest.
