Cycles materials are very different from the Blender Internal Renderer materials. It's time for a little introduction. You are going to make a texture that is partly diffused so that it absorbs light, and partly glossy so that it reflects light. This way, you can control how much light is reflected:
Use the RMB and press the Shift + RMB keys to select only the four sides of the last extrusion you made, as shown on the left of the next screenshot.
Press Ctrl + I to select the inverse.
In the Properties window, select the Material button in the header.
If the Material panel looks different, as shown in the center of the next screenshot, that's because you are now creating materials for the Cycles renderer.
Name the material Outside and assign it to the selected faces.
With the cursor over the 3D View window, press Ctrl + I to invert the selection.
Click on the + sign in the Material panel to make a new material. Click on the New button...