A blue sky is a contradiction: the sky at night is devoid of colour, so why during the day does the world seem to be shrouded in a blanket of blue?
Imagine... as you wake later than usual rolling over towards the window, you notice that it's a gorgeous day outside. Warm, yellow sunlight shines in through glass illuminating floating "dust angles." On the other side of the glass, past the oak tree with yellowing leaves, you see a brilliant blue sky. For the first time it occurs to you that a blue sky is a contradiction: the sky at night is devoid of color, so why during the day does the world seem to be shrouded in a blanket of blue? Years previously as a child full of questions you asked your parents, but the answer they offered seemed somehow inadequate at the time... less than magical. And so the question remains... as it does the most of us.
The answer is this: The sky isn't actually colored at all (not blue or yellow or red or green). Rather, it's your mind that's colored. The world around us is physics devoid of meaning, whereas our perception of the world is meaning devoid of physics. In terms of physics, the light in the sky is heavily biased towards smaller wavelengths (around 450 nanometers). This is because the air itself scatters smaller wavelengths of light more than it does larger ones. Which means the air in the sky is like a filter, letting primarily medium to long wavelengths through more easily than short wavelengths. Hence why the sky is composed primarily of shorter wavelengths (and so appears bluish), whereas the light from sun is composed primarily of longer wavelengths (and so appears more reddish). While the differential scattering of sunlight by the air explains the non-uniform distribution of wavelengths across the sky, it doesn't explain why shorter wavelengths are seen as blue and the longer ones as red.
The central squares on the upper and lower surfaces of this cube appear very different in color: Brown on the top and bright orange on the bottom. Scroll to the very end of the linked article to reveal their 'true' physical similarity.