A pretty word for "rainbow" could be "iridescence" or "prism". Both evoke the shimmering, multicolored beauty of a rainbow.

• Iridescence refers to the phenomenon of surfaces appearing to change color as the angle of view or illumination shifts, much like the shifting hues of a rainbow. It carries a sense of magic and wonder, often associated with natural beauty like the sheen of a pearl or the wings of a butterfly.

• Prism brings to mind the scientific elegance of light refracting through a prism, creating a spectrum of colors. It’s a word that feels both poetic and precise, capturing the essence of a rainbow’s formation.

If you’re looking for something more whimsical or poetic, you might consider "arc-en-ciel" (the French term for rainbow), which rolls off the tongue with a lyrical quality. Alternatively, "spectrum" or "chroma" could also serve as beautiful, evocative alternatives. Each of these words carries its own unique charm, reflecting the ethereal and enchanting nature of a rainbow.