A friend would like me make them a cake using whipped cream instead of frosting, because of Diet restrictions. Can it be colored? They want a guitar cake. So it would look a lot better if I could color it.

Yes it can, and you can add cocoa powder instead of food clouring or use choc ganache (whipped) if the guitar should be brown in colour

Here's a pic of coloured whipped cream (Harley Tank)

Just if you add any fondant or gumpaste pieces, remember to coat the underside with chocolate, whipped cream will start to disolve the pieces otherwise

Thansk!! and for the tip on the add ons I would never have thought about it.