My take on blue hair
I think due to personal experience I think the guy in the relationship is just plain useless.
"She asked me how to be funny
But that's not something you can teach"
What I interpreted was the girl trying to fit in and is asking on her partner to learn to be funny but the guy just doesn't know how to and doesn't even seem to try.
"What seemed so blue in the sunlight
By the night was a pale green"
I interpret this in 2 ways
first is that the girl has different identity in front of others but at night when they are alone she shows her true self and is much less confident or anxious and this is due to the wording of "pale".
2nd way is how the relationship seems mutual when the need to support each other comes up the mutualness seems to dissapear.
"And I tried to hold her
But it didn't really last long"
The guy genuinely wants to help but cant and the relationship doesnt last long
"And she's getting older
I guess she's gotta cut her blue hair off"
She is maturing and understanding that the relationship is not healthy and cutting off her blue hair for me is leaving the relationship.
"She asked me if she was pretty
Well, it's clear that the girl's a fraud"
She doesn't believe that she is pretty yet asks even thought the answer won't change her perspective of her self and I think that makes her a fraud.
"There's really no way of winning
If in their eyes you'll always be a dumb blonde"
He has a defeatist mindset and already comes to a conclusion that he can't help even when she is perceived as something she is not.
"And she cried over nothing
So there was nothing I could do to stop
Her from cutting
Her beautiful blue hair off"
Here he doesn't understand why she is crying over things cause he sees it as "nothing" and fails to show sympathy and help therefore can't do nothing but watch as she leaves
"It looked like cotton candy
And just as quick to get licked away"
the blue hair for me is the relationship and cotton candies are bright and blue in the air but dissolves easily in water. Which to me shows that in the "air" the relationship is fun they enjoy each others company but as soon as it needs to be serious and touches "water" it falls apart as it can't survive the not fun part of the relationship.
"Last I heard she was living
With a boy who acts his age"
She got into a new relationship with a boy who acts his age. Since adults or more mature people are often perceived as people who figured it out or is more knowledgable the boy can actually sympathize and help her.
"And I guess I'll just miss her
Even though she isn't even really gone
But things are just different
Ever since she cut her blue hair off"
This shows to me that even though they broke up they are still in some contact cause she isnt truly gone maybe stayed a friend but no longer a lover since she matured and left the relationship.
(Also english is my second language so sorry guys for grammar mistakes)
I think I interpreted it like this since I felt like I couldnt figure out and show sympathy in alot of my friendships.
(I am usually bad interpretting things so just feel free to flame me in the comments.)