From Ardrana

By means of the rainbow cantrip, the caster brings into being a plane of pastel colors that exactly resembles a natural rainbow. This 30-foot long, 1-foot wide band of color can he arched into a bowed shape, remain a ribbon, twist and turn, and so on. In any event, one end of the rainbow must be within 10 feet of the caster when the cantrip is cast.

The colors glow softly and are visible even in total (normal) darkness. If no motion is desired, the caster can leave the vicinity of the rainbow. If motion is desired, the caster must control it by hand gesture and remain within 10 feet of it. The rainbow lasts for but a minute before fading away.

Note: This spell originally appeared in this form in Unearthed Arcana for Advanced Dungeons & Dragons from TSR. Its use here is for the purposes of providing context for the campaign only.