Which ingredient is commonly used in hair color to lighten the natural pigment?

Hydrogen peroxide is widely used in hair color formulations for its ability to lighten the natural pigment in hair. It acts as an oxidizing agent that breaks down the melanin in the hair, which is responsible for its color. This lightening effect allows for the application of a new color and is fundamental in achieving the desired shade, especially when going from darker to lighter colors.

Ammonia, while also a common ingredient in hair dye, primarily serves to open the hair cuticle and facilitate the penetration of color into the hair shaft rather than lightening the hair itself. Alcohol is typically included in hair care products for its solvent properties and to assist in the drying process, but it does not lighten hair pigment. Glycerin is used for its moisturizing properties and does not contribute to lightening hair either. Thus, hydrogen peroxide is the correct choice due to its specific role in the chemical process of lightening hair pigment.