This can be explained in story not as lighting issues but actually due to Sun exposer lightening hair. . In ANH and TPM it Luke and Anakin have been living there whole lives on a desert planet with two suns. This would lighten their hair. In the next film they have been off Tatooine for a few years and the hair that has grown in the meantime has not been exposed to the same amount of sunlight so it has not lightened.