I'm inclined to think this is Palpita minuscula - the prominent medial discal spot on the forewing of P. annulifer (species group) is absent. Unfortunately the size range you give overlaps both species (minuscula = 17 to 20mm; annulifer group = 23 to 27mm).
Palpita minuscula has, to date, only been recorded from Hong Kong. The Type Locality is "Sai Kung Station" - i.e. Lion's Nature Centre / Tsui Hang SSSI, although I have also seen it in Lam Tsuen Valley (several sites); Sha Lo Tung; Tai Po Kau Headland (not in Tai Po Kau, though); Kadoorie Farm & Botanic Garden; Kadoorie Agric. Res. Ctr. and Dragon Garden, Sham Tseng. It seems to be a dry season species, with one record from August, otherwise October through April (n=17).
[ Last edited by hkmoths at 2008-4-14 10:53 ]