What is puzzling about the clouds is their color. The ammonia ice clouds should be white, yet they have a variety or red, orange, yellow, and brown colors. Sunlight striking the clouds causes photochemical reactions with the molecules in the clouds. The resulting organic compounds, or trace amounts of sulfur and phosphorus may be responsible for the colors in the clouds. If you are asking about yellow-greenish clouds to warn us about a tornado, this may not be true.

Experts don’t agree on why the sky changes color before a tornado or severe thunderstorm. At least two theories suggest what might be the reason for the strange greenish-yellow color that (sometimes) precedes a storm.

http://earthsky.org/earth/why-does-the-sky-change-color-before-a-tornado