The speaker compares love to a rainbow, a spectrum with more shades than the sky could ever paint. Each colour stands for a person who has given them love, and at one end sits the love of self. So the speaker admits a life full of many kinds of affection, and then asks the beloved not to measure or compare. "Do not, please, ask if I love you more" is the plea at the poem's emotional centre, and it sounds both tender and a little anxious.
The second half turns the image around. The beloved becomes a prism, but one that works in reverse: instead of splitting light into colours, it gathers everything into a single beam. All those shades fade into one steady light, linked with the stars, moon and sun, shining "forever and ever." The mood is warm, reassuring and devotional. The speaker ends by inviting the beloved to stop counting colours, close their eyes, and simply rest in the glow.