A DRS cluster becomes red when it is invalid. It may become red because failover capacity is violated.
The cluster attempts to failover virtual machines in case of host failure, but is not guaranteed to have enough resources available to failover all virtual machines covered by the failover requirements.
If a cluster enabled for HA loses so many resources that it can no longer fulfill its failover requirements, a message appears and the cluster's status changes to red.
Review the list of configuration issues in the yellow box at the top of the cluster Summary page and address the issue that is causing the problem.