肇論新疏 2.392

2.392

牒經以釋色即是空。故牒非色，初出正理。謂凡夫執青黃等相，皆謂實有者，不了從緣性空之理。故經破著，即於青黃色中求色無實，如幻如夢，故云非色於色。

Quoting the sūtra to explain that form is emptiness. Therefore, quoting "non-form," he first sets forth the correct principle. This means that ordinary beings cling to marks such as blue and yellow, all regarding them as truly existent, not understanding the principle that they are empty of self-nature through dependent origination. Therefore the sūtra refutes this attachment: seeking within blue and yellow form, form lacks substantial reality, being like an illusion or a dream; hence it says "it negates form with respect to form."