What visual precursor is most frequently reported before or during severe weather producing tornadoes or large hail?

Answer

Green

The most frequently reported and scientifically discussed color preceding or accompanying severe weather capable of producing tornadoes or large hail is green, even though some might expect a pitch-black sky.

What visual precursor is most frequently reported before or during severe weather producing tornadoes or large hail?
weathercolorskyatmospheretornado