You really shouldn't try this at home - invest in a hairstylist. Seriously. And I'm not one.
You can't lighten dark hair by using a lighter coloured boxed hair dye. The colour needs to be lifted (or stripped) by a hairstylist before the lighter colour is added.
A couple of years ago, when I was trying to save a few pennies, I began using boxed colour. For some unknown reason, even though it was red I was after (I'd had it for years), it turned dark. I mean DARK brown. I was horrified. I waited a few days, then purchased another box of red colour to try and fix it. It didn't work, stayed the same.
Found a good hairstylist who informed me you can't go lighter with a dark hair dye, only darker. He had to strip the dark brown, then reapply the red.
It looks great now. And although many people swear by box colour, I haven't ventured there again!
Hope this helps.