As brass wearing the semblance of gold, displays its own defect on being heated*, with a fire lit with Kusa, so you have betrayed your own lightness by this speech of yours. i.c. becomes dusky, (Ramanuja).
सर्वथा तु लघुत्वं ते कत्थनेन विदर्शितम्। सुवर्णप्रतिरूपेण तप्तेनेव कुशाग्निना॥