Move to the window. Look to the sky, what colour do you observe? I am sure the answer will be blue, yet when you look at the horizon in the evening at sunset, the colours are sometimes yellow and orange. What is the explanation for this?
A clear cloudless daytime sky is blue because molecules in the air scatter blue light from the sun more than they scatter light of other colours. When we look towards the sun at sunset, we see red and orange colours because the blue light has been scattered out and away from the line of sight, leaving the colours we see.
White light from the sun is a mixture of all the colours of the rainbow (Red, Orange, Yellow, Green, Blue, Indigo and Violet). These colours are distinguished by their different wavelengths, and the visible part of the spectrum ranges from red light which has the longest wavelength to violet which has the shortest wavelength while orange, yellow, green, blue and indigo are in between.
Also, light from the sun is not constant at all wavelengths, because some of them are absorbed by the high atmosphere, while others are scattered out before they reach the visible atmosphere, so there is usually more or less of each type of colour in the light that reaches us.
Further, our eyes are less sensitive to some colours like violet while the rest of the explanation lies in the way our vision works. There are three different types of colour receptors, or cones, in our retina – red, blue and green – because they respond most strongly to light at those wavelengths, and less strongly to light at other wavelengths thus giving us our colour vision.
When the air is clear, the sunset will appear yellow, because light from the sun has passed a long distance through air and some of the blue light has been scattered away. If the air is polluted with small particles like – dust, smoke, water droplets, or a mixture of these -, the sunset will be more red.
Now you can explain the different colour phases of the sky from day to night time.