How do I remap colors in Cycles? 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.
It looks like what you are looking for its the color ramp node.


The color ramp node can take any value between 0 and 1 (values outside this range will be clamped to 0 or 1) in it's fac input socket.  And it will output the respective color from the gradient specified by the color swatches just like in BI.

Use the + and - buttons to add more swatches.
Use the &lt;-&gt; button to flip the order of swatches.
The color (and alpha value) of the selected swatch is set with the large color strip at the bottom of the node.
You can adjust the exact position of the selected swatch with the pos spinner.
The alpha gradient is outputted through the alpha output socket.


So it could, as you mentioned, be used to adjust the affect of an image texture on a mix shader (the image texture could just get plugged directly into the mix shader if no color adjustment is needed though).