The reason why mugwort turns yellow after steaming is that chlorophyll is destroyed. When steaming mugwort, chlorophyll is destroyed, thus changing color. It will turn yellow or black, and Artemisia argyi is the easiest to change color.

Chlorophyll is a green pigment contained in higher plants and all other organisms that can carry out photosynthesis. Its structural feature is that it includes four porphyrin rings composed of pyrroles, and the four pyrroles are combined with magnesium. Chlorophyll exists in chloroplasts of leaves. In chloroplasts, chlorophyll can be considered to be embedded between protein layer and carotenoid lipid located beside chlorophyll phytol chain. When cells die, chlorophyll is released from chloroplasts, and free chloroplasts are unstable.