My son is six years old and starting to get his permanent teeth. I am concerned because the teeth are discolored. Some even have a brownish or greenish hue. What caused this and what can be done about it?

From your description, it sounds like a very normal situation and nothing to be concerned about. When the teeth are forming in the jaws, they are surrounded by a soft tissue membrane called Nasmyth's Membrane (named after Alexander Nasmyth, a Scottish dental surgeon in London who died in 1847). As the teeth erupt, remnants of this membrane remain on the surface of the enamel. The fibrous nature of the membrane readily picks up coloration from food. In most cases normal chewing and brushing will remove the remnants with time. If they don't come off, they can be removed by having a professional cleaning. Other causes for staining of teeth include high fevers during infancy, too much fluoride (makes teeth stronger) in drinking water and certain medications if taken while the teeth are still forming.