Can you make your hair lighter by using lemon juice?

Not exactly, Lemon juice by itself will not lighten the hair. Lemon juice AND the ultraviolet rays of the sun will lighten your hair. (Similar to a product called "Sun-In" ) The drawback here is dryness and if your is dark to begin with, it will turn orangutan orange! This job is better done by a professional. I also would like to suggest a "Weave". The weave (unlike the allover color) will not show such an extreme outgrowth line (dark roots). If you are already a dark blonde or very light brown you can probably use the lemon juice and sun method. Be sure to buy a really good "conditioning pac" for after. Also the lemon, sun method needs about 5 hours in the sun. (Not good for the skin so wear sun block!)