The aphid is a member of Uroleucon but the colour is very strange for that genus. Many species of Uroleucon are dark bronzy red or almost black. This is my first time seeing a green colour Uroleucon from HK. Looking through literature - the males of many species, however, are green, as are the females of some, particularly members of the Nearctic subgenus Lambersius and a related group of South American species, which live mostly on the tribe Astereae (American way of saying Compositae). Without specimens to study, it is difficult to provide a name with certainty.