Colours

Really really really long time back, when there weren't many people on earth, there lived two men.  I don't know their names, but let's call them Ganesan and Murugan.  When electromagnetic ration (aka light) with a certain frequency between 540 and 610 THz passed through Ganesan's eyes, he saw a shade of this color: .  However, for the same radiation Murugan saw a shade of .  Likewise, Ganesan saw the light of a certain frequency between 610 and 670 THz as  but Murugan saw the same light as .

Ganesan saw plants and leaves in , and seas and sky in .  Murugan saw plants and leaves in  and seas and sky in .  They spoke to each other about how there's something visually similar between sea and sky and between leaves and plants.  They decided to call that similarity "colour".  They decided that they would call the "colour of leaves" green and the "colour of sky" blue.  Keeping that as the starting point, they identified and named plenty of other colours.  But never could they know that one person's green was another person's blue.