Look at the beam both from the side and from the end, where the beam shines out of the container. From the side, the beam appears slightly blue, and on the end, it appears somewhat yellow. Now the beam of light looks even more blue from the side and more yellow, perhaps even orange, from the end. Add the rest of the milk to the water and stir the mixture. Now the beam looks even more blue, and from the end, it looks quite orange.
Light usually travels in straight lines, unless it encounters the edges of some material.
When the beam of a flashlight travels through air, we cannot see the beam from the side because the air is uniform, and the light from the flashlight travels in a straight line. The same is true when the beam travels through water, as in this experiment. The water is uniform, and the beam travels in a straight line. However, if there should be some dust in the air or water, then we can catch a glimpse of the beam where the light is scattered by the edges of the dust particles. When you added milk to the water, you added many tiny particles to the water.