It would certainly be a useful visual indication, but, for one thing, Cubase itself would have no idea where one layer ended and the next one began, so the best one could really hope for would be to customize velocity colors according to ranges that we could set manually (rather like the setup for Preferences>Metering>Appearance 
)… and, even if Steinberg did add that, there would currently be only one such color map per Project (or maybe they could at least allow to store color map settings.