I have made a model of a building. I was using the make face tool, but when I extruded it, it was a different color than the rest.
Its not any different material or anything either...
You need to reset your normals, go into edit mode, select everything A, and hit Ctrl-N.
This happens when blender incorrectly guesses which way the face normal should be pointing when you make a face, it happens from time to time.