Scientists' best theory for why the sky sometimes changes to a yellow or greenish hue before a severe storm is that storm clouds of a certain thickness and water concentration act as a filter for the light that passes through them, according to The Guardian. Green and yellow clouds were long considered to be an optical illusion rather than a true color change.
In 1995, a meteorologist named Frank Gallagher tackled this mystery for his thesis at the University of Oklahoma, says Scientific American. He used a spectrophotometer, a tool that measures the color and intensity of light, to take measurements from an oncoming storm, and found that the color of the sky did indeed change. There is still some debate as to why this happens. Some scientists thought that the heavy clouds might be reflecting the color of fields below, but Gallagher measured the light against a light green wheat field, and once more against the red dirt of a newly plowed field, and found that the colors did not match up. The theory that the clouds act as a light filter makes the most sense, but science has yet to find a direct link between this phenomenon and oncoming severe weather. Common wisdom advises people to seek shelter if the clouds above them turn a strange shade of yellow.