How do we know that light and dark blue are different shades of the same colour, but green and blue are different colours?

Question

Basically, how do we differentiate between different colours and different shades of the same colour.

In: 15

Answer 1

As people said its because we agreed to name them. My favorite one is the red robin, or redheads. They are both clearly orange. But there was no name for orange yet. So now we still call them red.