瑜伽論記 6.525

6.525

若小乘云變赤相者，死屍初青後赤變作赤血身以除形色貪，大乘解先彼皮肉等色赤，除去皮肉形色名變赤也。即變者赤也。

Regarding what the Hīnayāna calls the mark of turning red: a corpse is first blue-green, then red, turning into a body of red blood to eliminate greed for shape and form; the Mahāyāna explains that previously that skin, flesh, and so forth were red in color, and removing the shape and color of the skin and flesh is called "turning red." That is, what is transformed is the red color.