Capacity Planner - Why is a cluster headroom colour significantly different from the hosts in that cluster?

When you see a cluster showing a signficantly different colour from the hosts in that cluster, that is normaly the result of being constrained on shared storage.  Typical shared storage configuration is such that all hosts in a cluster have access to all datastores.  Therefore, when calculating headroom at the host level, the entire pool of shared storage is considered available to that host.  As a result, it is unlikely that shared storage will be a constraint on that host.  However, at the cluster level, the headroom of the cluster is determined as the percentage growth available across the cluster as a whole.  When this is considered, it is often the case that shared storage is the primary constraint.  It is possible to remove chosen metrics from headroom calculations by using the 'VM Headroom' tab on the left hand side and de-selecting thou.