The intense coloring of the molecule is generated by the absorption of specific
wavelengths of light by the
pi bonds. These bonds are ordinarily excited by light in the orange region of the spectrum, causing the molecule to appear blue. When the molecule interacts with
protons from an
acid the bonds become harder to excite and thus absorb green light which has a shorter wavelength. This is what causes the molecule to appear red in the presence of an acid. ==References==