According to The Guardian, scientists’ best idea for why the sky turns a yellow or greenish colour before a violent storm is because storm clouds of a specific thickness and water concentration function as a filter for the light passing through them. For a long time, green and yellow clouds were thought to be an optical illusion rather than genuine colour changes.
According to Scientific American, a meteorologist named Frank Gallagher explored this question for his thesis at the University of Oklahoma in 1995. He took measurements from an approaching storm using a spectrophotometer, a gadget that monitors the colour and intensity of light, and discovered that the sky colour did actually change. The reason behind this is still a point of contention. Some scientists speculated that the heavy clouds were reflecting the colour of fields below, but Gallagher measured the light against a light green wheat field and then against the red dirt of a newly ploughed field, and discovered that the colours did not match. Although the notion that clouds operate as a light filter makes the most sense, scientists have yet to discover a definite correlation between this phenomena and impending severe weather. If the skies above them become a weird colour of yellow, it is common sense to seek shelter.