Firstly, you need to understand how a regular rainbow works - when sunlight passes through a raindrop, it refracts, and white light is split into an arc of color. The wavelengths of light refract at different angles, which is why blue is always at the bottom of a primary rainbow.
But not only does the sunlight refract through water droplets, some of it also reflects. A double rainbow occurs when there is an extra reflection of light within the water drop. As some light is lost when it hits the edge of the drop, this secondary rainbow is fainter than the first, and also has a reversed color order.
So why are both the rainbows red in this image? The answer is Rayleigh scattering, which is the scattering of light by tiny particles in the atmosphere. This scattering is what causes the Sun to appear yellow and the sky to appear blue, and also results in red sunsets where the atmosphere is thicker around the horizon.
And because of this optical phenomenon, only the longest wavelengths of light of these rainbows - the red ones - are visible.
Reference:http://epod.usra.edu/blog/2014/05/sunset-rainbow-over-samos-island-greece.html
Articles:http://www.sciencealert.com/what-causes-a-double-red-rainbow
http://space.io9.com/a-double-red-rainbow-at-sunset-1589750573
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