Material Viewport Color

This entry will explain a simple concept of material identification, as well as touching on a special surface in Cycles called the Holdout.

FOR CYCLES:

In the initial viewport we have 3 cubes that have different colors. However, these colors are not representative of what the render will look like.

In cycles, in the material panel, you'll notice a field called Viewport Color.  This is what the object will be colored when viewed in Solid render mode.  In this example, the red is obviously the cube in the middle. You'll notice however, that the actual shader or surface used is Holdout.

When we view the rendered image, we see three boxes with three unique materials, none of which are colored.  The 'blue' box has a Glossy shader, the 'red' has a Holdout, and the 'green' has the Emmisive shader.  It's worth noting that the Holdout shader is essentially the opposite of the emmissive shader.  As the emissive shader emits light, the holdout shader actually absorbs all light and reflects none.