Hi sazzyg
There are so many pitfalls using box dyes, not least of which, you must perform a skin test beforehand. This is because you could suffer a serious reaction to the ingredients in the dye, which can be rather harsh in comparison to the products available to hairdressing professionals.
It would be best to return to your salon as they will know exactly what products were used originally and will be able to match your colour exactly.
With box dyes, there's a danger that your hair could turn brassy/orange or even take on a green tinge. If that happened, it would cost you a lot more money to have a hairdressing pro perform a colour correction service.
I'm not trying to be snooty, but hair colouring is not simply a matter of choosing a shade on a chart and bunging it on. If you really want to go DIY, then I'd recommend reading up on colour theory in a good hairdressing training manual. You can buy them from Amazon. I taught myself quite a lot from reading several course books before deciding to train professionally.
