Studies show that over half of us are not happy with the color of our teeth. Over the counter bleaching kits are in high demand because is cheaper than going to the dentist. Short term it can work but it can turn out to be more expensive in a long term, after the dentist tries to fix the damage done to the teeth by supermarket whitening agents.
The main component used in all bleaching systems over the counter or used by professionals is hydrogen peroxide. It is regarded by EU Health Organization and American Dental Society as the safest way of teeth whitening. Supermarket bleaching kits have a very low hydrogen of peroxide concentration, does not exceeds more than 0, 1% in EU, 6% in Australia and 12% in New Zeeland. Only professionals are allowed by the government regulation to use 25-35% concentration of hydrogen peroxide. Because of that strict control the companies are looking for alternatives like sodium chloride (the mechanism on the teeth is not fully explained) and citric acid that dissolve the enamel if used long term exposing the dentine (underlying layer) which is naturally yellow. This is an irreversible damage to the tooth structure, the enamel cannot be build back.
Gums also suffer from careless use of bleaching products, resulting in change of color and burning sensation.
Factors to be considered that influence the teeth shade: genetics, medication administrated in childhood (tetracycline), teeth trauma (an accidental face punch on the playground in the childhood can result in teeth darkening in the adult life), diet (coffee, tea, smoking, tobacco chewing), age, amalgam fillings called also silver fillings, etc.
Except factors mentioned above, keep in mind that existing fillings, crowns, veneers and other prosthetic work done in your oral cavity DO NOT change color, only natural tooth structure changes. A rainbow of white and yellow shades combination will not be very attractive so before using any bleaching system ask your dentist if it suits your type of enamel, esthetic outcome and if in generally it is safe to whiten your teeth.