Hue Falloff in Maya Lights

The basic idea is to calculate the "facing ratio" not of the camera's view of an object, but of the light's view of the object, and to map that light facing ratio through a ramp and back into the light's color attribute.

Here's a brief overview:

1) Get the world position of the point being shaded by using the samplerInfoNode.

2) Get the world position of the light using the lightInfo node.

3) Subtract (1) from (2) to get a vector from the surface point to the light.

4) Get the surface normal in camera space using the samplerInfo node.

5) Convert the surface normal to world space using a vectorProduct node set appropriately.

6) Take the dot product of (3) and (5) using another vectorProduct node set to "dot product" -- this is the light facing ratio.

7) Clamp the light facing ratio (6) to between 0 and 1.

8) Run the facing ratio intoi the V-coordinate of a V-ramp set to colors of your choice.

9) Loop the output of the ramp back into the light color.

You now have angle-based hue falloff built into the light.

To explore this in detail, download the maya scene file:
hueFalloffLight.ma

You can use any surface shaders you wish to use on the objects. All the color falloff action is in the light