妙法蓮華經玄義 4.18

4.18

次除卻皮肉，諦觀白骨。見骨色相異，謂青、黃、白、鴿，如是骨相亦復無我。得此觀時，名欲界定。

Next, removing the skin and flesh, one attentively contemplates the white bones. Seeing the colours and marks of the bones to be distinct—namely blue, yellow, white, and dove-coloured—such bone marks are also non-self. When attaining this contemplation, it is called the concentration of the desire realm.