your two chameleons will probably not fight. They will most likely just stress each other, and have one subtly dominate the other for resources until the other dies. The male will also breed the female too often drastically shortening her life.
Natural Selection and Geographic Isolation.
By selectively cross breeding, line breeding, etc. you can alter the appearance of these animals over the course of several generations, like anything else.
BUT the current effort is to preserve them as found in nature, primarily to avoid being a...