No one does not know exactly for sure. But a number of theories suggest what create the strange greenish-yellow color that can precede a storm.
To have a deeper look at the first theory, first you’ve got to picture mountains. Mountains from afar look a deepening blue. That blue color comes from the scattering of light by air molecules—it is the same reason the sky looks blue. Some experts believe that before a thunderstorm, golden-reddish light from the sun low in the sky and a natural bluing effect of the air blend to create a green sky. The storm provides a dark backdrop and offsets this greenish or yellowish hue.
Another meteorological theory holds that storm clouds themselves may help make the color of the sky bluish-green. It could be that the storm clouds—which are filled with water—provide the color blue, which is illuminated by the golden light of a low sun to create the color green.