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Light is made of electromagnetic waves. Different gases make different colours.
White light is all the colours of the rainbow.
Auroras are made of blue, green and red. There is lots of red mostly at the top of auroras, in the middle there is usually greenish white and a bit of blue/pink to finish it.
Different colours work better in different parts of the sky. Red works best in high and moderate altitudes, red also appears in oxygen , green is found in moderate altitudes, and blue is also found in moderate altitudes and appears in nitrogen. Blue is also a colour that is really common and scatters the most but purple scatters even more that blue but our eyes can't see that colour in the sky.
Colours that we can't are infrared rays and ultraviolet rays. These colours have too long of a wavelength to see.
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