It whitens hair by chemically reducing your hair's color pigment, or melanin. When exposed to the sun, the citric acid accelerates the bleaching process. The lightening effects of lemon juice tend to work best for lighter hair colors, such as blonde and light brown.Fresh lemon juice will not dry out or damage your hair. However, prolonged exposure to the sun can damage your hair’s outer sheath, called the cuticle. For this reason, you’ll want to limit the amount of time you spend in the sun after applying lemon juice to your hair.