肇論新疏 2.394

2.394

此釋前不非色於非色也。本就所執色中，非斥如幻以顯真空，故云非色。若非色於太虛，太虛本非色，何用更非？則非色名義自不成立。

This explains the previous "not that it negates form with respect to non-form." Fundamentally, within the form that is clung to, one refutes it as illusory to reveal true emptiness; hence it is called "non-form." If one negates form with respect to the great void, the great void is fundamentally non-form; what need is there to negate it further? Then the designation and meaning of "non-form" would of themselves not be established.