is not difficult.
Fill the soda bottle with water and prop up the flashlight so it will shine through it from the side. Now, add a (small amount) of milk to the water and shake the bottle to mix them up. Keep adding milk until you start to see blue light being scattered to your eye from the water/milk mixture. Just like the atmosphere, the mixture scatters more of the blue wavelengths that any other color.
At sunrise or sunset, there's even more scattering of the suns rays going on because they are now cutting at an angle across much more of the atmosphere. This causes even more of the blue and violet wavelengths to be scattered. What's left?-- The vivid reds and oranges that lie on the other sent of the color spectrum.
You can simulate sunset in your own blue sky by adding more milk until the side of the 2-liter bottle opposite the flashlight appears red. Look at the side of the bottle and you can see first the blue scattered to your eyes and finally the oranges and reds.
Why do you think the amount of milk in the bottle affects the color you see? And what happens to the color you see if you use very little milk or a lot of milk?
The red and orange colors of sunset become even more vivid when there's dust or smoke in the air -- for instance, when a volcano, forest fire, or dust storm puts up tiny bits of debris into the atmosphere. Forest fires sometimes put larger bits of soot into the air, which may scatter a different set of wavelengths and give you green sunsets or even blue moons.