It's a bit counter intuitive.
In the Calibrate Colors dialog there is a <Reset> button. The colour calibration is reset as soon as you click it, but if you then click <OK> it begins calibration again, so you have to hit <Cancel> instead.
It won't necessarily update the photo thumbnails either though, not straight away anyway, but if you open each photo you should see that the colours have been reset.
But to answer about the dense cloud, "Reset" won't update cloud point colours that have already been generated, but there is an option in Tools -> Dense Cloud -> Colorize Dense Cloud which will reapply the colours from the source images, although i don't know if this is much faster than regenerating the dense cloud (presumably it is!)