Why do white substances appear white?

In the context of biology, substances appear white because they reflect all wavelengths of the visible spectrum, rather than absorbing them.12

In biological structures such as feathers, mammalian hair, and flower petals, this appearance is often a result of structural coloration known as total reflection. This occurs when light is scattered by finely divided materials separated by minute air spaces. Additionally, deposits or secretions within tissues—such as the proteins and fats found in milk or the calcium carbonate in mollusk shells—can contribute to this effect.3

While some organisms possess specialized pigment-containing cells called leucophores that appear white, the perception of white itself is an achromatic sensation that occurs when the human eye is exposed to a combination of all wavelengths of the visible spectrum simultaneously. Unlike pigmentary colors, which are of molecular origin and absorb specific light wavelengths, the whiteness produced by structural reflection is a physical phenomenon involving the scattering of incident light.1453