This liquid has multiple nutrients, and perhaps one of them creates that characteristic white color. A detail to remember is how skimmed milk doesn't evidence the same color of whole milk. Then, it's a good indication to associate the white color with at least grease. With a more specific specification, the first reason are fat globules.
Fat globules are in the category of intracellular grease, and it's interesting that if these globules are smaller, the effect is more white color due to the dispersion of light. When the milk is taken directly from a mammal, it normally has a subtle tone toward yellow, but after the process of homogenization the globules get smaller.
After other two compounds are also affecting the color, casein and colloidal calcium phosphate. Casein is a well-known protein attached to phosphoric acid and for this reason receives the name of phosphoprotein. Perhaps a bit technical for everyday use, so let's stay with casein as a protein that provides amino acids, calcium, phosphorus and carbohydrates.
With a great nutritional content of course, milk can be enough for sustain life when it specially demands energy for growing properly.
And what about calcium phosphate? This compound enters in the realm of minerals, and it's not by chance that our bones are made of calcium in a high percentage.
All as always with versatility and usefulness in the analysis. Milk has thousand uses for cooking, giving also variations like cheese, yoghourt, creams. Pastry and so on, and a delicate taste that varies from different areas, but it's always white!