What does the observation that early dead galaxies were already red suggest about the timing of their quenching?

Answer

The quenching process was a relatively rapid event

The visual transition to redness, marked by the death of short-lived blue stars, indicates that the shutting down of star formation occurred quickly rather than being a slow, drawn-out decline.

What does the observation that early dead galaxies were already red suggest about the timing of their quenching?
Spacegalaxiesastronomydead galaxies