What will lemon juice do to light brown hair?

It depends on how much lemon juice you use and how dark brown your hair is. If you put it into a squirt bottle and just spray your hair (then blow dry it or go out into the sun) you'll end up getting blonde streaks or just lightening your own original color. I've done this and it works pretty well. If you soak your hair in it then it your hair will be a lot lighter (than if you used a spray bottle). Unfortunately there isn't a set color when it comes to lemon juice and it often takes a few days to a few weeks to finally figure out what the final color is. If you're trying to have a specific color then you're better off with buying a thing of hair dye. Also forewarning, lemon juice makes your hair very dry and brittle. Hope this helps! :-)