The red pigment in hydrangea sepals is the flavylium cation of delphinidin-3-glucoside. When hydrangea shrubs grow in acidic soils, aluminum ions are assimilated into the plant and are transported to the sepals where they complex with delphinidin-3-glucoside, resulting in blue coloration. Such color changes were modeled by mixtures of Al(III) and delphinidin in an acidic ethanol solvent system. The mechanism for the bluing requires Al(III) to extract a proton from a flavylium cation of delphinidin, which then rearranges to its quinoidal base form that complexes with the Al(III). In addition, the remaining flavylium cations of delphinidin undergo a systematic bathochromic shift. The percent complexation of the delphinidin {that present in the quinoidal base form} and absorption wavelength of the two forms of delphinidin were monitored as a function of the concentration of metal ion. The effectiveness of bluing of delphinidin in this model system is aluminum > tin > molybdenum > zirconium >> iron > gallium and erbium. Both tin and molybdenum are effective bluing agents, although not as effective as aluminum. The final blue entity, no matter what the metal ion, forms when the flavylium cation of delphinidin self-associates with the complexed quinoidal base form. Accordingly, the limiting percentage of the quinoidal base form of delphinidin is 50% in solutions with metal ions in great molar excess of the delphinidin.