Danger of the same stupidity as when they had a black Vulcan, only perhaps worse. As per the typical SciFi cliche vulcan is a desert planet, or at least the part that Spock came from. Then again most of the vulcans we saw in the original series came from the same region, so perhaps as far as vulcan goes it's only mildly sunny.
In the case of Galifrey, the whole of the TimeLord society lived inside a domed city. Other Galifreyans left from there. It seems that so much interbreeding would have taken place that they'd all end up with the same skin tone.
However there is always the Romana option, where she adopted the appearance of someone she'd met in her travels. Hugh Quarshie would then be eligible for the role.