Someone may have already said this ( but I am too lazy right now to read ) but first you need to strip the hair. You can but the correct chemicals at a proper beauty supply store and then it needs to be treated ( special conditioner ) and then you should be safe to dye it. I would advise leaving a few days between stripping and dying although not necesary.
How do I know this? Cos when I went from black to blonde once I went via green first. :shock: