Yellow teeth while psychologically distracting are otherwise a natural function of maturity. With age, the teeth naturally darken. Whitening teeth with bleaching gels can be of value in lightening up teeth by several shades. All modern bleaching gels contain an organic form of peroxide gel. They vary according to their percentage. As you can imagine, a greater strength of peroxide gel will be more potentially caustic to the teeth and gums. In my experience, gels over 20% will often cause a burning sensation to the gums and cause tooth sensitivity for a time. I say "often" because some people don't seem to get significant burning or sensitivity. My best advice here is to seek out a lower percentage organic bleach (my office uses 14% bleach) and see if it gives you a result you can be happy with. If not, then opt to try a higher percentage bleach. I've seen bleaches as high as 35% on the market.