The authors regret to inform that the link http://prodotti-speciali.it/wp-content/uploads/2015/02/gradazioni-300x204.jpg at pag. 311 of the publication results no longer active. The color reference provided by the link should therefore be substituted. To do so, we reconstructed the color scale from the Figure 1, as follows: The tool “color picker” from the Adobe Photoshop suite was applied to the html of Figure 1. top right, i.e. the 36-color scale proceeding from a commercial website for dentist supplies. The tool returns color specifics in different systems; among them HEX and RGB were selected. The table below reports HEX and RGB values for each one of the 36 levels of the scale. [Formula Presented] The authors would like to apologise for any inconvenience caused and thank Dr. Filipa Bessa for alerting about the missing information; Dr. Leonardo Bertelli for the help with graphic information.