One way to do it would be to just use a texture map with two different colors on it. In your example, you'd just use a texture map that's green on top, red on the bottom. I think that there are also a couple of different material modifiers that will allow you to do a blend between top and bottom, but with a hard line between them. There's also a material type called Top/Bottom which would do what you're looking to do. Of course, if you want to map just a single face, you'd have to assign a material ID to it.