Using ash blonde dye to lighten dark brown hair

It will lift it a few shades, but will probably reveal a lot of warm tones rather than going an ashy or neutral brown. (Depending on the colour of your natural brown)

Thanks for the reply. Should I go for a very light blonde or a dark blonde?

The very light blondes often contain a bleach sachet so will lighten more. A darker blonde will be a safer option. The actual shade isn't going to matter hugely as it won't show up over darker hair, but anything ashy will help to tone the warmth out. Do try it on a strand before committing to your whole head, to make certain your hair won't be too orange.

That happened to me!  About four years ago, I wanted to lighten my hair a few shades from my dark, rich, chocolate brown colour.  Since it was very dark, I thought a blonde box dye would bring it down a few shades to start, and I'd end up with a lighter brown.

I must have bought a "very light blonde" that apparently had bleach, because - after using the colour late one Sunday night - I had no time to fix my hair for the morning and showed up to work as a blonde, and all eyes were on me, I was so self-conscious!  That Monday night I immediately covered it up with my dark brown again because I didn't mean to lighten it so much.

Now I have gone blonde, but this time it was intentional.  So just make sure you know, as Wicked Pixie says above, that those "very light blonde" box dyes will probably contain bleach and just may lighten much more than you realise!