Oddly enough, coloring my hair is the best way to control the psoriasis on my scalp! It does much better at controlling it than anything the doctors have subscribed. My hair stylist has told me that he knows others with the same experience. An odd, though positive, side-effect :)
@kjay, aveda does use the same developers as all the other color systems out there. if you are just doing color, you don't get the fumey smell like you would if you were doing bleach or a high lift blonde.
The American Cancer Society's summary of current research suggests that risk is very limited: http://www.cancer.org/docroot/PED/co...X_Hair_Dye.asp