Orange sky

This is a picture I took of a sunset in Firenze. The sky was bright orange - not a hint of blue.

The reason that sunset is often orange is because the areas to the west stole all the blue.

The daytime sky gets its blue color from Rayleigh scattering. Most of the violet light is scattered across the sky, but our eyes aren't sensitive to violet, so we see the next most scattered color - blue.

But, as we can see from this next diagram, although the path the light travels through the atmosphere at noon is short, at sunset it is considerably longer.

By the time the sunset light reaches us, the blue and violet has been scattered out. What's left is green-orange-yellow-red. When our eyes see green and red at the same time, they actually perceive yellow. So, sunset starts as a yellow and becomes more orange and red as green is scattered out of it.