I find this an interesting bird. I don't think this can be either of the common Munia species (scaly-breasted or White-rumped). The plumage does not fit for any plumage of these - overall plain in colour but with strong rufous tones to the upperparts (especially nape) and a pale belly. Also this bird has a relatively heavy two-tone bill, blue eye-ring and fairly square-cut tail. I considered a juvenile White-headed (given the number of records of this species recently) or Chestnut, but those don't seem to fit either.
The bird is clearly an estrildidae species (munias, parrotfinches, etc.), probably a munia. There appears to be some contrast in the wing feathers (greater coverts and tertials) in terms of pattern and wear, suggesting it is either adult or moulting into adult plumage. There is no gape line (which is often very obvious on a juvenile munia), again supporting this being an adult. Assuming it is not an extreme mutant of one of the HK munias, it is almost certainly an escape. Beyond that, I'm really not sure what it is!
