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I know how to assign a color ramp to a single dimension (I.e. How to make a color gradient from red to white along the x axis)
What I am stuck with is how to make a color gradient based on a circular rotation.
For instance, if I had a circle, 0 would be red, PI/2 is yellow, PI is green, 3pi/2 is blue with the gradient working its way around.
You can set a Gradient Texture to Radial for this. Here i combined it with the Object Output of the Texture Coordinate Node, but depending on your geometry you might have to put a Mapping node behind it.
Looks like you'll have to convert to polar coordinates using the math nodes. Doesn't sound fun, but it is possible. Here's the equation.
θ = tan-1 ( y / x )
Actually that would be around the z axis, but you get the idea.
If you need help figuring out how to do it check out this tutorial from blender cookie https://cgcookie.com/archive/custom-uv-transformation-nodes-cycles/
