I wonder if the red might be intensified with a bit of Beta-Carotene supplement. Store-bought chicken has Beta-Carotene in it. Chickens are fed color enhancers to make their flesh pink. This could be a dead end, as I'm sure many indigo owners feed store-bought chicken to their snakes, but Beta-Carotene does bring up color in pink coachwhips and many lizards.

Your photo shows a magnificent specimen, with a perfect face--no rubbing on that one. Outstanding animal and evidence of good care.