They have color coding defined:
- Green = No reports of COVID-19 or COVID-19-like illness (CLI) for 7 days, as determined by a qualified medical professional
- Orange = Cases reported in less than 0.10% of passengers (e.g., if 6500* passengers are on board, it would take at least 7 passenger-cases during the previous 7 days to meet CDC’s investigation threshold), and no crew cases reported.
- Yellow = Cases reported in 0.10% or more of passengers (e.g., if 6500* passengers on board, CDC’s investigation threshold is met if there are 7 or more cases among passengers occurring during the previous 7 days), or one or more cases reported in crew.
- Red = sustained transmission of COVID-19 or CLI, or potential for COVID-19 cases to overwhelm on board medical center resources.
So as I understand this, which should not be taken as any kind of gospel, it depends on what the onboard medical center can handle; there is further detail about links between cases, number of ventilators available, and vaccination percentage of passengers. I would assume that at Red, they turn the ship around, and Yellow may also trigger that as it triggers CDC investigation.