Provided are coumarin compounds of formula I, for example, robustic acid methyl ether, and the use of such compounds and compositions thereof to modulate (e.g., inhibit) melanogenesis and pigmentation. wherein R3, R4, R5, R6, R7 and R8 are described herein. Also provided are plant extracts containing a compound of formula I, and the use of such a plant extract to modulate (e.g., inhibit) melanogenesis and pigmentation. The compound or plant extract may be prepared as pharmaceutical and cosmetic compositions, and may be used for the prevention and treatment of conditions that are related to aberrant melanogenesis activity.