it's an interesting point this un', and not one I entirely disagree with you on, but....
For one thing, it's not just about skin colour - it's about having ordinary working-class people standing, peoples occupations are also part of the 'look'. Secondly, although their is obviously no direct link between peoples skin colour - or income, or class - and their politics, having a broader range of people standing is likely to mean that a broader range of people are getting their views heard as well. I mean, no one would want to have a city hall full of white-middle-class men, even if their politics were ace. And if a national (or semi-national, or whatever) organisation can't find a pretty good cross-section of the public to stand for them, then I would be suspicious of their ability to actually represent everyone, and their commitment to doing so.