<prompt> According to the Nathoo classification, the chromogen changes color after binding to the tooth is categorized under:N1 type stainN2 type stainN3 type stainN4 type stain  <response>: 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 