Subscription status in Content Hosts

I'm ignorant, but going to training for Satellite server next week. In our installation we have a number of RHEL Server 5.11 hosts, some of which show up in Content Hosts as a red subscription status and other green. Irrespective of red or green, they all show only 1 installable errata (bug fix).

What determines whether a given host is red or green?
If they are red, will installable errata still be installable?