When I first read it, I believed and still believe the poem to mean that the narrator was doing something one way, what was comfortable to her "everything I had known". But then she tries something different and has a whole new outlook, one with color "blue-white", the narrator made a change on a "moving " sea. And now she is happy, because things ar no longer bland, "gray" or colorless- things are the "opposite." She is happy and sees the world in a new way , in a new color.
You can apply this to any character that changes in a novel. Here, Isla was stuck in a house with her family and only saw the small towns that she lived in. But at the end of the book we get to see that she has taken a travel that Bo had told her about, a big adventure. She is seeing the surrounding new lands and animals in a new color and she is thankful to Bo for introducing this to her, she never would have had this beautiful moment without him and it's definitely different than everything she had ever known before.