Menu --> View --> "Show out of range color"
The only downside is that this won't work with VSTi's it doesn't know about. :)
(So with external VSTi's, or using Studio One as a sound source, this might not work at all and so far I couldn't find a way to set the range yourself for each and every instrument *and* for each and every expression used. As out of range is usually dependant on the expression you're using, a set range will not work and it HAS to look at the current expression in use AND the instrument.)
