A DRS cluster becomes red when it is invalid. It may become red because the resource pool tree is not internally consistent.
If the cluster resource pool tree is not internally consistent (for example, the sum of the children's reservations is greater than the parent pool's nonexpandable reservation), the cluster does not have enough resources to satisfy the reservations of all running virtual machines making the cluster red.
This can occur if vCenter Server is unavailable or if resource pool settings are changed while a virtual machine is in a failover state.
Revert the associated changes or otherwise revise the resource pool settings.