Final answer:
Protonated ABTS is greenish-blue in color due to the absorption of light in the green spectrum and reflects the remaining colors.
Step-by-step explanation:
The color of protonated (non-radical) ABTS is known to be greenish-blue in solution. This color change occurs due to the presence of an extended system of conjugated pi bonds in the ABTS molecule, which absorbs light in the visible range, particularly at a maximum absorption (Amax) of 524 nm. When ABTS is protonated, the molecule absorbs light that falls within the green region of the spectrum, therefore when we look at the solution it appears greenish-blue to our eyes due to the reflected light.
As an analogy, this behavior is similar to that of acid-base indicators like methyl orange or phenolphthalein, which change color in response to the pH level of the solution. Just like methyl orange transitions from yellow to red when going from basic to acidic conditions, ABTS changes its color upon protonation. Methyl orange's transition is due to a shift in equilibrium in response to changing hydronium ion concentrations as dictated by Le Châtelier's principle.