Stay away from hair lightening products such as Sun In.
Most contain some form of peroxide or metalic salt and can cause severe damage to your hair, not to mention such undesirable effects such as orange, green, or pink hair. Even worse, if the product does contain metalic salt and you then perm your hair, it may literally turn your hair into mush and wash down the sink.
Although a chelator, (an ingredient in some shampoos), can help remove some of the metalic salts residing on the hair shaft, they are not strong enough to remove metal deposits that have penetrated the shaft. The only chemicals that we know of that will do this are chemicals used in the development of film. These chemicals are highly toxic and we do not recommend that you mess with them.
Bottom line, just let nature do its own thing, or invest in a highlight. In the long run the highlight will be far less expensive.