'Above us only sky', sang John, sorry Mr Lennon, the 'sky' does not exist.

The ability to see colours presents distinct selective advantages for species. Such as being able to recognise predators, food and mates.

 Indeed it is thought that, our bodies and brains have evolved to ‘create’ colour. We have evolved within limitations to have the colour system we have because it has allowed us to function well in the world. 

Plant life has been important to our evolution and so the ability to place in one category the things that are green has apparent value for survival. 

 The same goes for blood and the colour red, the sky and the colour blue. We have the colour concepts we have because of the physical limitations constraining evolutionary advantages

.What is "blue", after all?

 So-called "color" may be biologically-constrained, but is ultimately a cultural construction. 

Saying that the "sky" is "blue" is "true" because of the

 Other cultures have no such concept as "blue"; thus, it can't be an objective fact about the "sky", or anything for that matter, that it is "blue". 

We walk around with cave person ideas? 

Think John Lennon 'above us only sky'

 from that globalist anthem 'Imagine'

Sorry John, there is no Sky, Imagine is one of those cultural constructions 

that makes one believe in a "sky"?