It might go ginger if you're not careful and you have to be in the sun a long time. It's unpredictable depending on your hair type/texture/how long you stay out/how much you put on.
sorry bout that, but what i was trying to say was that it works best in blondes, and in all honesty it probably wouldnt really make a very drstic change on a dark brunette, your best bet is with using a product designed for lightening brunette hair or going for highlights
I'm not sure if lemon juice would work on dark brown hair. Naturally, my hair is light brown hair and when i tried the lemon juice/vodka thing years ago it just made it go a dark orange colour! Also, it ruined the condition of my hair big time and took weeks of intensive conditioning to get it back to normal. I agree with Roxy about getting highlights using a product designed for dark hair.
if you don't stay out very long then there probably won't be much colour change at all although it may make your hair very dry. the ginger colour is the same thing which happens if you attempt to dye dark brown hair blonde with a product designed for much lighter hair which does not contain enough peroxide.
I also have brown hair and two years ago I put lemon in my hair And stayed out in the sun for a whole summer. My hair got lighter but now it's totally messed up now, I really regret doing it. And I warn other people to not try it.