Who says that leaves are green

[Cassiopeia88]

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

Busker, I assume you are a very literal and scientific person, aren't you? Well, this is a poetry forum, so I wasn't talking just about the scientific facts, it's a little more complicated/philosophical/metaphorical than that. Poetry doesn't have just one interpretation, each person can find something different in each piece. To answer some of your questions:
1. Yes, the poem from my point of view talks about perceptions, the notion of change and asking questions.
2. Of course blue and green ARE different. I actually did have physiological perception in mind here + leaves change colors during the seasons, sky is different during the storm/sunset/etc., while we usually just say "The sky is blue". You can also ask yourself why we give colors certain names, if our perception is the only true reality ...
3. Well, the use of imagery in poetry usually does not correspond just to the literal reality, but also good vs. evil or other dychotomies. We can go scientific here and talk about infra red radiation, Sun eclipses, midnight Sun or whatever, or you can take a more metaphorical approach, whatever one prefers.
4.Thanks for the comment, I appreciate the rational approach here.

---

Everyone, thanks for your comments. I rewrote the poem, which is now way longer. I'd appreciate some thoughts, I think I kind of prefer the original version though

Who says that leaves are green
or sky is blue?
Who says that days are always light
or night stands immanently dark?


Who says the Sun will rise at East
or the Earth is flat and still?
Who says that history is made of facts
or fiction, myths are never real?

Who says that one must lead the pack
or that we're all the same?
Who says the path you followed once
will never shift its way?

Who says that all is set in stone
or that questions should be asked?
Who says one's role is to obey
or to be different than the rest?

Who says what's right
or what Right is?
Who states the Truth
or what we need?

Who says I have to be like you?
Who says you have to be like me?

For all that I do know

your green may be my blue
my light may not be light for you
my truth may be your lie to me
and our colors change as seasons go.