Color on paper is from pigments that reflect certain colors (wavelengths) of light and absorb others. Blue pigment, for example, reflects blue light and absorbs other wavelengths. Bleach is an oxidizing agent, meaning that it produces a chemical reaction that is oxidative with pigments. In the most general sense oxidation means taking away electrons, though that is often accomplished by bonding oxygen atoms in a chemical reaction, hence the name. By oxidizing the pigments, bleach changes the colors (wavelengths) of light the pigment absorbs. When something is bleached white, it reflects all colors (wavelengths).

On a molecular level, the electrons that interact with certain wavelengths of light by absorbing photons no longer do so because the pigment's chemistry has changed. It is electrons mediating molecular bonds in particular that interact with light, hence these bonds are changed such that they no longer absorb light when a pigment is bleached.

Thank you for your question!Common bleach that you use in your house for laundry and cleaning is mostly just a solution of a salt called sodium hypochlorite, which has the chemical formula NaClO. The reason that we see colors is because dyes and pigments contain molecules called chromophores, which reflect some wavelengths of visible light (these are the colors that you see). The chemical structure of the chromophore is important in its ability to appear colorful. Sodium hypochlorite (which has oxygen in the chemical formula) is a strong oxidizer, and it breaks up the bonds in the chromophore molecules. One the bonds are broken, the chromophore no longer reflects the visible light you saw previously, and the color disappears.

Bleach (with the technical name sodium hypochlorite) is a moleculeformed out of sodium, chlorine, and oxygen -and this trapped oxygenis the important bit when we think about removing stains and changingcolors. When bleach is in water (the liquid bleach we usually thinkof), it falls apart to generate hydrochloric acid and atomic oxygen.This oxygen is very reactive, and can disrupt and break the chemicalbonds of other compounds. How does this lead to a color change? Mostcolored substances (paper, clothes, stains, and more) gain theircolored nature from molecules called chromophores, which absorb lightof certain frequencies (different colors). In fact, a blue T-shirtappears blue because it is absorbing all the light except for bluethat strikes it, letting blue light bounce off and into our eyes.When the atomic oxygen from bleach reacts with the chromophores, itdestroys them, breaking chemical bonds. This usually results in thesubstance becoming colorless. This destruction, combined with theeffects of the acid also generated from bleach, serves to disinfectand clean up problem stains, and remove colors from many things.