Demi permanent hair dye over permanent?

I would advise using a 50/50 mix of vosene and fairy liquid, it will make your hair dry but I’ve found it great for pulling out colour. Stripping your hair with more chemicals would probably thin it.

What you then put on it depends on what outcome you are looking for, do you want to keep it the violet tone or are you looking for a more natural tone?

Have you thought about trying one of those colour strippers? They do really work. I've used them a couple of times because I can't get the semi-permanent dye I used to use. I've stripped the old dye off and put the new dye on. It won't take off ALL the old dye but would take off enough to let you then put a demi on.

The hairdresser would say that (to gain a regular customer) wouldn't they? A semi-permanent should be fine while you grow it out. Choose something closer to your natural colour. It will gradually fade and wash out.

Your other option is to have a permanent colour to match your roots, but the regrowth of grey may be more visible.