Waersen's poem is a short, plain-spoken look back at a happier time that the speaker feels is gone for good. The first lines are full of color. The speaker remembers a mind filled with the "brightest" colors and lines, which suggests a time of joy, imagination, and hope. The rainbow in the title stands for that feeling, and "summer" adds warmth and ease.
Then the mood turns. The speaker admits to having "took my life for granted," a line that carries regret, as if the good days were only appreciated after they ended. Now the speaker says they "suffer" a life of sadness. The short lines near the end, "All that was then, / Now is gone," feel blunt and final, like a door closing. The last line brings back the rainbow, but as something lost. The poem leaves a quiet, wistful mood, with a speaker who is looking back with sorrow and some self-blame.