I'd like to remap colours in Cycles the way one would have done in Blender Internal, such that I could take the output of the shader/material and change the range of colours to something else, say from a ramp. This was how Blender Guru was able to change the texture map on his Realistic Earth so that the dark side of the planet had one texture, while the other did not. Such a technique could also be used to affect the fac input on Mix Nodes, as well as parameters, such as Glossy 'Roughness'.
I've tried many things without achieving the results I desire, but I'm still new to Cycles.