Light that looks white to our eyes is actually made up of many different colours. Each ray of light or light wave is of a different wavelength or size. The shorter waves seem to be blue while the longer waves seem red in colour to our eyes.
What makes the Sky change colours?
When light comes from the sun, the light waves of different lengths travel through the atmosphere. These light waves come into contact with particles in the earth’s atmosphere- like dust, ice crystals, water droplets. The light waves then bounce off these particles and get scattered here and there in the atmosphere.
What determines Sky’s colours at Sunrise and Sunset?
Particles that are small in size as compared to the light waves scatter blue light. Red light waves are scattered by gas molecules of nitrogen and oxygen in our atmosphere. Thus we see the blue sky most of the time. But when the light waves have to travel a great distance, the blue light is removed. So during sunrise and sunset, when the sunlight travels a really long path to reach us, the blue light is removed leaving mostly orange, yellow and red lights. This light is reflected by clouds and other objects and makes the sky a beautiful, colourful reddish, orange one!
Thus the light colour depends on the size of the particles compared with the wavelength of the light.