Normal dye after henna

[madcapped]

I henna'd my hair about a month ago and to my utter disappointment, it failed to cover any of my copius greys.

I have quite dark coarse frizzy hair anyway and the greys are worse. I was hoping that the henna would work and also smooth everything out a bit especially since it was such a faff. I also prefer a less red tone to my colour

I can't afford a salon job so I guess the question is how soon can I go back to box dye and what would be the best one to go for?