What a question? Thanks team for your thoughts. To me it seems to be a unique mixing of the colours from our sunlight. We know that the blue sky is because of the blue part of the spectrum of colour that comes from the white light from the Sun.
Some of the blue part of that spectrum of light is scattered when it hits the molecules of Oxygen and Nitrogen in Earth’s atmosphere. So blue coloured light illuminates the daytime sky. We know that when we see a sunset we can see a red sky in the distance and that is because we are looking at the Sun through a lot more atmosphere than we would do in a normal day. This is the dust that is in the atmosphere. In the experiment the milk acts as a ‘dust’ in the water.
I attach a NASA video that explains this then I will tell you my ideas about a green sky.
My thinking, and that of some others, is that the green sky is linked to the storm clouds (the background image above, was a pause in the hailstorm that the cyclists were experiencing). These were preventing some of the sunlight reaching the viewer who saw a yellow light mixed with the blue sunlight. the mixture of these lights could have caused the green light (a cyano type of light, like that below).
Very happy to hear comments and questions. Remember science development is about admitting your ignorance and my thoughts above are just thoughts. You and I need to do some further investigations.
Revised 10/1/18 to include experiment and colour example.