Pink and Blue start out as two children who look alike and want to be friends. As they grow up and go to school, the other Pinks and Blues pull them apart, and the two do what they're told even though they don't want to. The poem's plain, simple wording makes the sadness feel real: each one spends the days wondering whether the other is okay.
The turning point comes when Pink realizes that Purple is a beautiful color. By joining together, Pink and Blue become something new, and the others disapprove. Purple doesn't care and is finally happy.
The colors seem to stand for groups people are sorted into, and the poem suggests that friendship or love across those lines is worth defending. The speaker sounds hopeful and a little defiant. The mood moves from loneliness and longing to quiet confidence, ending on the firm, almost stubborn line that Purple "is a fine color."