Verbs about appearance:
* Arcing: The rainbow arced across the sky.
* Stretching: The rainbow stretched from horizon to horizon.
* Shimmering: The colors shimmered in the mist.
* Fading: The rainbow faded as the sun climbed higher.
* Dancing: The rainbow danced in the spray of the waterfall. (more poetic)
Verbs about behavior:
* Forming: The rain stopped and a rainbow formed.
* Dissolving: The rainbow dissolved as the sun broke through the clouds.
* Appearing: A rainbow appeared after the storm.
* Disappearing: The rainbow disappeared as quickly as it had come.
Figurative language:
* Painting: The sun painted a rainbow across the sky.
* Smiling: The sky smiled with a rainbow.
Remember: These verbs are more about describing the rainbow's visual impact than the rainbow itself performing an action.