Chemical Concept Demonstrated: Reversible oxidation-reduction reactions
When the indicator is initially added to the solution, the solution appears green. The color slowly fades to produces a yellow solution, passing through an intermediate stage in which the solution is red. The green color reappears when the flask is shaken and then fades to red, then yellow again.
Explanations (including important chemical equations):
Indigo carmine undergoes a reversible reduction.
Shaking the flask brings more oxygen into the solution, provoking the formation of the oxidized (green) form. The indigo carmine will eventually undergo reduction again to its reduced (yellow) form. The intermediate color is due to formation of a red semiquinone intermediate.