How to highlight parts of my scene - strategy for materials? I have a scene that looks like this...



The renderer is Cycles (probably obvious from the screenshot :) ). Blender 2.78.

I'd like to be able to highlight components, such as the gearbox, in a highlight color such as a slightly emissive orange. This would be for stills and for animation; and for animation it would be cool to be able to fade from current materials to highlighted material over a few frames.

The main question I have relates to materials strategy. Currently, I've got materials such as aluminum, shiny_steel, powder_coat, etc. So this means that if I want to highlight, say, the gearbox, I need all the objects in the gearbox (which are more than one material) to change color, without affecting everything else in the scene that also has the same materials.

At this point, I don't have a handy hierarchy as these meshes were imported more-or-less flat; but I'd be happy to parent all the gearbox parts to an empty called "EMPTY_gearbox", for example. Then maybe the ideal thing would be to add a custom property to the parent that allows me to fade from assigned materials to highlight material, perhaps through a driver (I'm kinda guessing here - I'm not an expert yet... :) )

What's the best strategy for achieving this? How do I get my head around material slots, mix shaders, etc.? I'd rather not have to have several different aluminums, such as aluminum_for_gearbox, aluminum_for_manifold, etc. But if this is what it takes, I can do it, but it seems like a clunky approach.

Thanks!

Dan
I'm answering my own question as none of the answers was quite what I was looking for, but there were gems in there that let me figure it out. Thanks to all those who answered!

My solution:


I forked local copies of materials, e.g. Aluminum is now Aluminum.Gearbox, Aluminum.Manifold, etc. Thankfully, no component has more than 4 materials, so this was much less painful than I was thinking it might be.
I created my orange highlight material (some glossy, some diffuse and some emissive), using a node group to make it easy to place in all the materials where I want it.
I added a mix shader node before the output of these forked materials, and hung the orange color on the other input.
I added an empty, such as "EMPTY_gearbox_highlighter" with a custom property of "Highlight_Strength", going from 0.000 to 1.000.
I added simple drivers on the Fac of the mix shaders for all the materials in the gearbox, to be directly controlled by the Highlight_Strength property on the empty.


I'll repeat this for each highlightable component, and then all I need to do is to animate the Highlight_Strength properties on my empties, and it'll fade all the materials in the gearbox (or other component) to orange, which is exactly what I want!

Thanks to all who gave me the stepping stones to get here!

Here's the gearbox unhighlighted:



And here it is highlighted, just by taking that single custom parameter to 1.000: