It is just a matter of simple physics. The reason why items appear white is because they reflect all light wavelengths and absorb none. If all wavelengths of light were absorbed and not were reflected back, the object would appear black.
The color white is nature's light buffet, in that it results from all the wavelengths of visible light being reflected into the eye. Casein and certain fats reflect wide ranges of wavelengths, causing milk to appear white.
Milk is nature's buffet for babies, providing all the nutrition a mammalian infant needs for early development outside the womb. Cow's milk provides a range of nutrients for humans, although intolerance to lactose — a difficult-to-digest sugar — is a common affliction.
Milk consists of about 5 percent lactose, 3.7 percent fats and 3.5 percent proteins. Calcium-rich casein is by far the most common protein, and it is the combination of casein and certain fats that give bovine juice its color.
While milk is primarily composed of water (approx. 87%), which is a colorless liquid; there are other components of milk (fat, protein, lactose, minerals and vitamins) that influence the colour of milk. The fat and protein molecules in the milk reflect light at a wavelength that makes the liquid appear white.
While milk is rich in calcium, in the last few years, studies have highlighted the negative side to dairy, be it the sensation of bloating, the carcinogenic hormone content in milk, or weakening of bones caused by it.
If we were to remove some of the fat from the milk (like in skim milk), it would give the milk a different colour because of the wavelengths the light is reflecting back to our eyes. That is why skim milk has a bluish tinge.
