The color of the ribbon was said to be periwinkle some years back when it was discussed on these boards. So I really thought it was periwinkle. But searching on the net now, I see most places refer to purple as the correct color.
I have a periwinkle ribbon (made it myself), but it is no use in using it if the color is purple.
Maybe a double band would be a great idea. There should be one color representing autoimmune disease in general, and the different types like sjogrens, lupus and so on, should be marked with a narrower ribbon on top of the base color.