It's a Green-veined White Pieris napi. Because of the contrast between the almost white butterfly and the dark background, the yellow on the underside of the hindwing is all but lost. However, the dark bands on the veins are very clear.
The dark spots on the forewing are also good for P. napi.
Also, P. napi is smaller than P. brassicae, but of course they don't hold still for you to measure them.
Interesting point re the 'green' colouration on this and some other butterflies...
It is an optical illusion. The scales are actually a mixture of black ones and yellow ones, which, if done a certain way, fools our eyes into seeing green. Because humans are not natural predators of butterflies, one presumes the optical illusion is also effective in their predators, such as birds.
Some lepidoptera do have truly green scales, but in many such species the colour fades to a pale yellow or straw shade; emerald moths are an example.