Question 7 - Why is the sky normally blue, sometimes red/orange/yellow-ish, sometimes indigo/violet-ish but NEVER green?

I'm afraid the questioner is incorrect. The sky is green when illuminated by the northern lights (aurora borealis). But that's due to oxygen flaring in the atmosphere rather than scattered sunlight.

Tempting as it is to stop there, I'll also attempt a proper answer.
Sky colour is more normally caused by the scattering of light. Consider this - if light was not scattered by our atmosphere, the sun would be a blazing light in the sky and everything else would be black (other than the dots of stars). In every other direction, we would be looking into the void of space and see nothing but blackness.

Obviously this does not happen.

Instead, sunlight hits our atmosphere and the light is scattered and bounced and generally buggered about with by the oxygen and nitrogen that hangs around (when it's not being breathed by creatures like us). It just so happens that blue light gets kicked around dramatically more than any other colour so it gets sprayed out in all directions. From the ground, looking up, that makes the whole sky look blue.

Red light, and other colours, come off the sun and shoot straight through so they are part of the blazing shaft of light coming straight from the sun to your eyes.

At sunset/sunrise, the sunlight has to grind its way through more atmosphere because of the angle of the sun in relation to you, the observer. Since it's going further, most of the blue light gets scattered out in random directions, leaving the red colouring - with the majority of the sky still blue, especially away from the sun.

Green doesn't really get a look in - it's not scattered enough to show up against the blue and what doesn't get scattered isn't really noticed amongst the red around a sunset.

As ever, it doesn't pay to sit on the fence by occupying the middle ground. The glory goes to the colours at the ends of the spectrum.

There probably could be a more rigorous explanation but it won't be written here by me tonight (or ever, probably).