Description:

The rarest color of lightning among the options is green lightning.

Here's why:

Purple lightning can appear during intense storms, especially when there's high humidity or certain atmospheric particles; it's unusual but not the rarest.

Red lightning often refers to sprites, a high-altitude phenomenon that appears red but is more frequently observed than green lightning.

Green lightning is extremely rare and often associated with volcanic eruptions, where certain gases and conditions cause this unique coloration.