Just made a batch of red buttercream...what a pain that was. Does anyone charge extra for icing a cake with mostly red icing? As a gift, I am making a Lightening McQueen. I tried to talk the woman out of her idea but I couldn't get through to her.
I too would try to dissuade anyone from a cake with lots of red or black icing. As you say, it's a pain to make, tastes funny and stains everyone's mouth. I made a Laser Quest vest cake for my son's birthday which was covered in black fondant, and I made the kids peel it off before eating the cake. They were hyper enough as it was without needing all that extra colour!
As for charging extra, well, I guess that would be one way of talking customers out of it! It does take extra colour, as well as extra work, so you could justify it that way.
I only use dark/bright red or other dark colors as borders on cakes. If the main part of the cake needs to be red (or other deep/bright colors) I use an airbrush to spray white icing with airbrush color. You don't need much color and it doesn't end up tasting nasty!