Cropped

I was shooting a white cabbage butterfly (Pieris rapae) foraging on Saliva plant, but when I looked at the picture at home I saw another butterfly on the top of the inflorescence. I never have seen such a green butterfly. I have not being able to identify what species it is. It looks similar to white angled sulphur butterfly, but Indiana is not a range where the species is common. The thick veins that are typical for the species are not seen on the wings. Does anybody know what species the butterfly belong?

On the left side of the collage: the white cabbage butterfly is on the bottom. On the right: the enlarged picture of the unknown butterfly (the same as on the left).