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.