This is the beautiful Lemon Migrant butterfly (Catopsilia Pomona) ; this one found just the other week above me in Sai Kung late one afternoon.
They are a vibrant, striking butterfly. Their wing colour can vary from yellow to creamy white, with dark margins. This one was much darker than many I have seen, and confused me on the ID for a while.
Their caterpillars are a well camouflaged green colour with creamy white lines along each side.