Who says that leaves are green

[busker]

Who says that leaves are green
Or sky is blue?
Who says that days are always light
Or night is immanently dark?
 
Who says what anything is really like?

Who says I have to be like you?
Who says you have to be like me?
For your green may be my blue
My light may not be light for you
 
… and my colors change as seasons go.

What is the point that the poem is trying to make? Presumably, that the perception of “reality” is different for different observers. 
Blue and green correspond to distinct wavelengths and so ARE different (of course, you could question whether the perception of length is the same for everybody but evolution would have selected a conforming group). So the question becomes one of whether the perception of blue and green are physiologically the same for all, which we have no way of confirming.
In the second instance, the narrator says that nights are not always dark and days not always bright. For this to make literal sense, I’ll assume that the reference is to infra red radiation, or other forms of sensory perception, which can be better at night with less background “noise”.

The second stanza is repetitive, but otherwise ok