You need to remove those leaf stems called petioles, because they hold water. Leaving them attached can cause mold or disease. Pistils will turn colors under two kinds of conditions. Natural or unnatural conditions. Natural conditions are genetics, humidity, or dew point. and temps or age. After a period of time, all pistils turn color. Unnatural conditions are too much water or fertilizers. Light too close or too hot and dry or cold and wet. Right before mold will break out the pistils will turn brown.