Oh! I have to reply to this too I do my own foils etc and am naturally dark brown, so I know this works! If you want an alternative to purple shampoo, try mixing some toner with your conditioner, you can even apply it to dry hair (but not for as long as it'll be too porous). I use directions white toner, it's about £2.50 a pot and can last me a month as a weekly colour refresher

I like olive oil. warm some in the microwave, massage it through your hair then wrap a microwaved towel around your head for as long as you have time for. Shampoo.You can also leave it on overnight if you protect your pillow from the oil.
I have also used mayonnaise, mayonnaise is basically eggs and oil.

Use of these oils is highly recommended in ayurveda as these are easily absorbed in the body. These can be easily re-applied on the body for fast relief in case of requirement. High valued tulsi and other oils are used in curing cough, pain, cold and various other ailments.