The different colours are a result of the different charged states of the dye molecule. In the red form, all three nitrogen atoms carry a positive charge. The two sulfonic acid groups have extremely low p"K"a and will normally be negatively charged, thus at a pH of around zero the dye will be a cation with an overall charge of +1. The green colour corresponds to a form of the dye with no net overall charge. In neutral media (pH 7), only the nitrogen atom of the diphenylamine moiety carries a positive charge and the blue dye molecule is an anion with an overall charge of −1. The p"K"a for the loss of the two protons are 1.15 and 1.82 respectively. The final proton is lost under alkaline conditions and the dye becomes pink in colour (p"K"a 12.4).