俱舍論頌疏論本 28.14

28.14

無色謂無色者，無色界中，謂無色故，立無色名。依大眾部、化地部等，無色界中許有色法，以色微故，立無色名。

Regarding "The formless is called so because it lacks form": within the formless realm, because it is said to lack form, the name "formless" is established. According to the Mahāsāṃghika, Mahīśāsaka, and other [schools], form is accepted to exist in the formless realm; it is established with the name "formless" because that form is subtle.