Description:

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

Explanation:

• Purple lightning: Often seen during intense thunderstorms, typically when lightning interacts with atmospheric particles, creating a mix of blue and red hues.
• Red lightning: Often associated with "sprites," which are electrical discharges occurring high above thunderstorms in the upper atmosphere.
• Green lightning: Extremely rare and typically observed during volcanic eruptions (volcanic lightning). The green hue may be due to the interaction of lightning with certain gases or extreme atmospheric conditions.

Since green lightning is almost never observed in regular thunderstorms and is mainly seen in rare volcanic lightning events, it is considered the rarest.