Why Milk Is White

The short answer is that milk is white because it reflects all wavelengths of visible light. The mixture of reflected colors produces white light. The reason for this is due to the chemical composition of milk and the size of the particles contained within it. 

Chemical Composition and Color

Milk is about 87% water and 13% solids. It contains several molecules that don't absorb color, including the protein casein, calcium complexes, and fats. Although there are colored compounds in milk, they are not present in a high enough concentration to matter. The light scattering from the particles that make milk a colloid prevent much color absorption. Light scattering also accounts for why snow is white.

The ivory or slight yellow color of some milk has two causes. First, the vitamin riboflavin in milk has a greenish yellow color. Second, the cow's diet is a factor. A diet high in carotene (the pigment found in carrots and pumpkins) colors milk.

Why Skim Milk Is Blue?

Fat-free or skim milk has a bluish cast because of the Tyndall effect. There is less of ivory or white color because skim milk doesn't contain the large fat globules that would make it opaque. Casein makes up about 80% of the protein in milk. This protein scatters slightly more blue light than red. Also, carotene is a fat-soluble form of vitamin A that is lost when fat is skimmed, removing a source of yellow color.

Summing It Up

Milk isn't white because it contains molecules that have a white color, but because its particles scatter other colors so well. White is a special color formed when multiple wavelengths of light blend together.