Lush henna help

I stopped using box dyes ages ago because it was wrecking my hair and I was having to colour it every 3 weeks for the grey coming through. 

I was using Surya Brasil Henna cream which was brilliant but blooming expensive when you have almost waist length thick hair.  I then made a fatal error - instead of going for my usual light brown, I decided to go with ash brown (because in my world, ash brown is a cool tone and would cool down the reds) and I went with the powder this time.  *boo*

Cue Kevin the Carrot.

So, thought I would try Lush Caca Brun - even redder than Kevin the Carrot and whilst I could live with it if it was even - it's not and it certainly didn't do anything for my greys.

So off I trotted and got the Caca Noir.  I've just strand tested it and it really doesn't seem to have done much and I've no idea what I'm doing wrong - it's just not taking on my hair at all.

Any tips for the caca noir?  Would having conditioned my hair after washing stop it taking?  I've read to put salt in with indigo to help it - but how much?

I really need to sort out all this glowing red