Have two lamps blend together to give another color on a face I am working through Blender 3d Basics by Gordon C Fisher. I'm on page 72. I have a green light and a red light but no yellow as pictured. Can anyone explain what I am doing wrong? Or better, What to do to get the yellow color on the cube? It is black where the yellow should be (on top of cube), and was also black in the previous two with the one red light and two red lights.

How could I have two lamps shining on adjacent sides of a cube (1) and (2) blend together and create a different color on the top face (3).
You more than likely have the default point lamps at the exact middle of these faces, therefore they won't reach the top. Raise it above the top of the cube so they can illumiate the top face. The lamps will blend naturally and give you the third color.