demi permanent hair colour not lasting

[Gatekeeper]

I only have a very small amount of grey hair around my hairline but I really don't like the look of them so have been using Casting semi on this section of my hair only. Have noticed of late that it doesn't last very long anymore- certainly not the up to 28 washes it promises

I don;t really want to use start using a permanent and wondered if anyone has had the same problem and found something that lasts a bit longer? I'd far rather use something that faded rather than have obvious root regrowth but need something that lasts more than two weeks

ta!

I use Castings for the same reason although now at 42 I am starting to develop a smattering of grey roots along my parting too.

In between dying I use this spray l'oreal root spray

I find it does the job well but I have dark brown hair. Not sure if the lighter ones are as good.