Use free photoscenery images to derieve the colour of building roofs
That way the colour variation is closer to reality and does not need the existing parameter for distribution of roof colours. However, this means a large amount of disk space would hat to be ready, an algorithm defined to get the roof colour - and only certain free data could be used. If there are residuals in the photoscenery (e.g. clouds, colour fading etc.), then still a correcting mechanism would have to be used. Is it worth it?