. Why do poles have to come in pairs?
There don't appear to be any isolated poles in our universe, or at least none have been found. That's just the way it is. As a result of this situation, the only way to create magnetism is through its relationship with electricity. When you use electricity to create magnetic fields, you effectively create equal pairs of poles—as much north pole as south pole.
. Would placing a blue filter on a Xerox machine prevent it from making copies, since blue light has more energy than red?
No. Blue light causes the photoconductor to conduct. When you use white light in a xerographic copier, it's the blue and green portions of the light that usually do the copying. The red is wasted.