Mostly, the poem is about noticing that someone's happiness isn't real. The speaker begins by remembering how bright this person seemed: a smile, a golden laugh, and a world that felt entirely yellow. The images that follow, wildflowers, sunsets, endless sunny days, are warm and pleasant, almost too perfect.
Then the tone shifts. The world "melting like an ice cream" and "swimming in thick syrup" turn the sweetness into something sticky and heavy. The speaker seems to sense that all this cheerfulness takes effort, comparing it to "painting a perfect grin." A painted smile is a mask, and holding it up is exhausting.
The final line lands the point. Yellow stands for joy, and blue, in the speaker's view, stands for the sadness underneath. The mood begins glowing and dreamy, then slowly grows uneasy and tender, as the speaker finally sees the person behind the smile.