Best hair dye for over dyed hair?

[Lovemusic33]

I'm 35 years young but very grey, I dye my hair every 6 weeks at the moment and it still doesn't cover my greys for more than a couple weeks. I don't want to dye it more often as my hair is becoming thin and dry but I don't want to look like I'm 50 years old either. I have tried the dyes with no ammonia but they don't seem to take to my hair.

Can anyone recommend a hair dye with good coverage that doesn't wash out?

I get on well with Olia- do my roots every four weeks with it. Trick is to do the roots, give it a scrunch with the mid lengths of your hair after 20 mins and leave for 10 more mins.

(I have dry, fine hair so sounds quite similar)

I tried the permanent version of Naturtint from H&B a while ago and found that although it was nice and gentle on my hair, the colour faded and lost its brightness after a few weeks which isn't ideal for permanent dye! May be better now though, I haven't tried it for a good five years.