Milk is a homogeneous rather than heterogeneous mixture. The reason for this is that the different chemical components that comprise milk are not visibly separate.
Under a microscope, milk can be considered heterogeneous because the lipid (fat) cells that give milk its white appearance are not actually dissolved into the water that makes up the rest of the substance. However, milk retains its status as a homogeneous substance because when viewed macroscopically—that is, when seen with the naked eye, it appears to be a solution without notable separation.
In addition to being a homogeneous mixture, milk is also a colloid. For a substance to qualify as a colloid, the size of the particles comprising the mixture must be larger than individual atoms or molecules, but not so large as to be visible to the naked eye. The dispersed particles in a colloid are between 1 and 1,000 nanometers in diameter.