All milk is white but the shades of white will vary depending upon the composition or what is dissolved in the milk. The most important factors that contribute to the color of milk are fat, how much cassein there is (one of the classes of milk proteins) and what calcium complexes (these are minerals), and how much water-soluble riboflavin (a vitamin). These compounds contribute to milk's opaque or ivory color by interfering with light transmission.
So why is milk
white and not
green for example? Because there are so many fat and protein molecules in milk that enough light at all wavelengths gets scattered giving the appearance of white light. If all the wavelength of the light would get absorbed, milk would be black.
The color also depends on what the cow eats. If there is a lot of carotene in the diet it will also affect the color. They provide a yellowish color. High concentrations of riboflavin also contribute to the yellow color. Removal of the fat and therfore carotenoid pigments and solids when making skim milk makes the milk look bluish.
UCSB Science Line