Yogācārabhūmi Śāstra, Fascicle 34 (13)

Original text:云何寻思内事显色所作。变异无常之性。谓由观见。或自或他先有妙色。肌肤鲜泽。后见恶色。肌肤枯槁。复于后时。还见妙色。肌肤鲜泽。见是事已。便作是念。如是诸行。其性无常。何以故。此内显色。前后变异。现可得故。

Explanation: How does one contemplate the impermanent and mutable nature manifested in the visible colors (rūpa) of internal bodily matters as perceived by the eye consciousness? One observes that oneself or others originally had beautiful complexions, with skin fresh and lustrous, but later one finds the complexion has deteriorated, becoming withered and dry, no longer fresh and lustrous; after a period of time, one again sees the complexion become beautiful and the skin moist and lustrous. After observing these phenomena, one thinks in one's mind: formations such as these are indeed impermanent in nature. Why is this said? Because these visible colors within the body differ before and after, changing and impermanent, and this can be observed directly in the present.