Common Emigrant
Catopsilia pomona

I saw this species commonly in Kolkata and elsewhere, as well (I think) as its close relative the mottled emigrant, Catopsilia pyranthe. In fact, I think it was the first butterfly I saw, fequenting avenues of trees in the centre of the city. But despite being common it was very mobile and hard to photograph - the picture above is the only half decent picture I got.

Fred had similar difficulties with the species, but got this single shot:

My butterfly is the pale yellow-white of a male while his is the deeper yellow of a female.