At the risk of stating the obvious, membership of the Green Party in Brighton is not a cross-section of the UK population. It's not even a cross-section of the Brighton population being, from my observation (not just from this picture, but from local Green Party people I've met), overwhelmingly middle-class. It's not at all surprising therefore that (in Brighton anyway) they're going to be pretty much overwhelmingly white. I certainly don't think that makes them racist, and most of them seem a well-meaning enough bunch, although you might wonder how in touch the Green Party members are with the needs and concerns of working class people, disadvantaged people and people from ethnic minorities.