I'd read that it's very hard to go over certain colours with the new Contrast paints. How did you find that? Did you use the recommended primer?
All the colours that I used covered pretty well. Some are heavier than others, though. I used the Wraithbone spray on some of them (including the mushroom man above), but for others, I just painted over white or preshaded white undercoat. The demon above was done over white that I'd washed with Agrax Earthshade and drybrushed white again. There's not much difference - but the yellow feathers would have been brighter without the preshading.
The stronger/darker paints 'cancel' the lighter ones very well. So if you're painting light to dark, you can erase your mistakes as you go along. But cleaning up's not that difficult, as you can paint over any mistakes in the Wraithbone pot paint and use contrast paint again. There's very little in the way of 'tidemarks'; they seem to dry not at all for a bit and then suddenly, rather than drying gradually.