When a single renderer.color is changed in a group of objects, change the remaining objects.

I'd suggest looking at the answer to this question: http://answers.unity3d.com/questions/25392/public-variable-listener-for-dispatch-in-composed.html

Why not just give all the cubes that need to change color a shared material. That way, changing the material color changes all of the cubes with that material.

The documentation doesn't make it clear to me how shared materials work and how to use them.

ins$$anonymous$$d of setting renderer.material = your$$anonymous$$aterial; (which creates a new instance of the your$$anonymous$$aterial for each object its on)

you would set renderer.shared$$anonymous$$aterial = your$$anonymous$$aterial;

Now each object that you set to that shared material will reflect any changes you make to that material, i.e. the color you want to change.

To expand on the shared$$anonymous$$aterial mentioned by @brianruggieri. When changing the shared$$anonymous$$aterial, you are modifying the 'base' material permanently. Create a few object and add the same material to each. Then add the following script to one or more of the objects:

 #pragma strict
 
 function Start() {
     renderer.shared$$anonymous$$aterial.color = Color.red;
 }
 
 function On$$anonymous$$ouseDown() {
     renderer.shared$$anonymous$$aterial.color = Color.green;
 }