Milk is white because of its reflectance properties. The reason anything is white is that it reflects all wavelengths of light and absorbs none, therefore it is white. If milk were black, that would mean it absorbs all wavelengths of light and reflects none. The reason a shirt is blue is that all wavelengths of light are absorbed except for blue, which is reflected.
Light can have other properties as well. For instance, the reason that asphalt on the road gets hot is because it's black and absorbs almost all energy from the sun and reflects almost none. White things, like roofs, tend to reflect wavelengths of light and therefore keep the house cooler.

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.

NEAT question. Do you have any guesses? Why is raspberry juice red? Well, something in the raspberry gives it the red color, and when you make juice from raspberries the red stuff goes into the juice too, right? Milk is white, right? Well, milk is white because there is something in the milk that makes it white. There are proteins, fats and carbohydrates (a big word for types of sugar). All of these make the milk look white.

For scientists and future scientists, there is a more complicated answer. Milk is white 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. Maybe you will want
to send Scienceline another question about light scattering, or maybe you will want to ask a friend, teacher or parent to explain it. Anyway, it doesn't change the fact that milk is tasty, right?

I don't know exactly why milk is white. In general, though, the color of something depends on what colors it reflects versus what colors it absorbs or transmits. So, if you have "white" light (all colors of the rainbow) shining on, say, lemonade then yellow light is getting reflected to your eyes while other colors are being absorbed by the lemonade. If you can see through the lemonade, then some of the light is also being transmitted through. You usually can't see through milk so very little light is getting
transmitted. The light that get reflected looks white. The rest is adsorbed.