So first of all this question likely suffers from the same problem as this question. I say likely, because I was never able to make the answers for that question work, so I can't be sure exactly what is going on. I AM fairly certain that I simply don't know some crucial part of applying materials, and that is causing this problem.
The problem is that if I try to apply a material to an object, the material is only applied to some faces. In the linked question, I tried to apply a material and a texture to a roof structure, so I decided to scale things down and practice on a cube. Same problem.
I created a simple green material, but it only applied to two out of six faces. Selecting the whole cube and clicking assign again has no effect. The rest of the cube is black.
Quite simply, I don't know why this is happening. Based on my understanding of materials, when you select a face and hit assign, the material is assigned to that face. Either this isn't happening, or it just isn't showing up. Why is this? What am I missing?
If possible, I would like an explanation of how the material works, not just a simple 'click this then that' answer.
NOTE: I apologize if this seems too much like a duplicate of the linked question. I'm asking again because the answers supplied never fixed the problem for me, and because I wanted to remove all other factors and work with a simple cube for reference.