Making a model appear to be black and white?!?

How would one do this? I have been trying to think of a simple way to make the model appear black and white (without colors) without having to assign a black and white texture to it.

Any ideas?

Vertex colour?

I don't there's any way to do that other than use a monochrome texture. It wouldn't be *that* hard to create a function which turns any texture into it's monochrome equivelant.

I always think its a shame we don't have HSV screen modes so we can have effects which mess with saturation etc.