He said that EPIV was all about earth tones, your browns and grays etc, EPV was about metal and tech, so white, black and silver, and EPVI was all about a return to organics, hence the Ewoks beating the Empire, with a lush pallette of greens and browns.
When you think about it, kind of makes sense. I'm not sure if the Prequels were given this much artistic thought, but there it is...