Why Milk is White?

Although milk has a simple appearance, it has a complex chemical structure and composition. The vast majority consists of water, but it has a nutritious structure with oil, protein, minerals, and other compounds.

The reason why anything is white is that reflects all wavelengths of visible light and does not absorb any of them. In milk, proteins form structures ranging in size from 50-500 nm in diameter (average about 120 nm) called micelles. This minimal micelles can scatter light that hits them. Therefore they contribute to milk's opaque or white color by interfering with light transmission. The color of milk may also be affected depending on the animal's diet and the fat content in milk.
If you have further interest in milk, you will take pleasure in investigating the bluish color in milk :)

Phadungath, C. 2005. Casein micelle structure: a concise review. Songklanakarin Journal Science Technology