俱舍論疏 1.209

1.209

論「何故此蘊至說為色耶」，問色名也。上來釋五根、五境無表為後，皆是色蘊。釋色既了，須識色名，故今問起。

The Śāstra says: "Why is this aggregate... called form (rūpa)?" This asks about the name of form. Above, explaining the five sense faculties, five sense fields, and lastly the unmanifest, these are all the form aggregate. Having finished explaining form, one must understand the name of form; hence this question is now raised.