The speaker is in love with a close friend, and each color becomes a small piece of that story. Blue and purple hold favorite colors and happy moments, like laughing together and dancing with burning hands. Other colors carry the hurt: grey is mixed signals and failing to understand, red is watching the friend with someone else from a wooden seat, and orange hints that the friend's cheerfulness might be a facade.
The order of the colors seems to trace a timeline, though that's an inference. Green looks back to how it all began in homeroom, while yellow suggests the speaker leaving, and that things are "not the same." The speaker still clings to hope, even while looking away and looking at the friend in the same breath.
The final rainbow pulls everything together. It's the friendship, the speaker's feelings, and an open ending: the friend can make of it "whatever you make of it." The mood is tender, wistful, and quietly vulnerable.