This morning I died my hair medium mahogony (permenant) and it is now really dark. I washed it about 3 times with anti dandruff shampoo. Will this help lighten the colour and if so, how long do you think it will take? I have heard that hot water also aids in stripping colour - does this apply to using a hair dryer or does that help set the colour?

Over the next week or so it will gradually lighten, but don't keep washing it like the way you are, over washing will leave you with dry scalp, just leave it and it will lighten out itself with the sun and gradual washing.