Milk is a type of 

 Milk is a solution of fat and protein dissolved in water.

A colloid is a type of solution in which the size of particles is not visible to naked eyes but they can scatter the beam of light. A colloid solution appears to be homogenous but it is actually a heterogenous mixture.

Milk is a type of colloid solution as it has fat and protein dissolved in water and these solid solute particles are not visible but they can scatter the light.