Milk is a colloid, which contains tiny particles of fat and protein. Mixed with water, the particles scatter light much as dust scatters light in the atmosphere. Light is scattered differently, depending on its color or wavelength. Blue light is scattered the most, while orange and red light are scattered the least.
Light travels in a straight line until it encounters particles, which deflect or scatter it. In pure air or water, you can’t see a beam of light and it travels along a straight path. When there are particles in the air or water, like dust or ash light is scattered by the edges of the particles.
Looking at the daytime sky is like viewing a flashlight beam from the side — you see scattered blue light. Looking at sunrise or sunset is like looking directly into the beam of the flashlight — you see the light that isn’t scattered, which is orange and red.
What makes sunrise and sunset different from the daytime sky? It’s the amount of atmosphere the sunlight has to cross before it reaches your eyes. If you think of the atmosphere as a coating covering the Earth, sunlight at noon passes through the thinnest part of the coating (which has the least number of particles).
Sunlight at sunrise and sunset has to take a sideways path to the same point, through a lot more “coating”, which means there are a lot more particles that can scatter light.