The Lemon Emigrant (Catopsilia pomona) is a butterfly that exhibits seasonal dimorphism: they look different depending on the season.
In summer or the wet season there is the form 'crocale', in winter or the dry season the form 'pomona'.
The form crocale has the antennae black and the form pomona has the antennae pinkish or red. 
The males are easily identifiable, even in flight, on account of their overall lemon colour. 
Identifying females is harder, due to their many different colour forms.
Some females have reddish-brown rings or patches and/or black spots near the wingtips.
The Lemon Emigrant is found in Asia and parts of Australia.

Form 'pomona'. Cat Ba National Park, Cat Ba Island, Vietnam, 6 & 7 January 2024