How to add driver to emissive color in Cycles? I want to add a simple one-line driver to control the emissive color of a material in Cycles.  I had assumed I either hover over the color and press 'D' or right-click and select 'Add Driver' from the context menu.

Neither of these options worked, so I searched around and found some forum posts that suggested finding the emissive color datablock in the outliner, but that did not work either.

Lastly, I tried the same shortcuts in the Node view of my Cycles material network.  That also didn't work.

In all three cases I tried both right clicking the color AND right clicking the RGB values inside the color inspector.

I feel like I must be overlooking something extremely simple here, as driving a color property seems like a common task -- a natural next steap after animating a color property with keyframes.
I am an idiot -- if the value is already keyframed you are not able to add a driver to it.  Once I deleted all the existing keyframes I was able to add a driver.