According to the Nathoo classification, the chromogen changes color after binding to the tooth is categorized under:

N1 type or direct dental stain:  The colored materials (chromogen) bind to the tooth surface and cause  discoloration. The color of the dental stain is same as the color of the chromogen. N2 type or direct dental stain:  The chromogen changes color after binding to the tooth. N3 type or indirect dental stain:  Colorless material or a prechromogen binds to the tooth and  undergoes chemical reaction to cause a stain. Textbook of Endodontics Nisha GARG 3rd ED