Instance portal

An instance portal is the sparkling field that marks the entrance to an instance.

The color of the portal indicates the type of instance:

Blue Instance Portal: Group (5 or less)
Green Instance Portal: Raid (10 or more, on a raid reset timer)
Red Instance Portal: PvP Battlezone (battleground). These portals are no longer functional with the addition of battlemasters at the entrances.
White Instance Portal: Transit. Currently, the only such portals are the Deeprun Tram entrances, the portal between the Ghostlands and the Eastern Plaguelands, and the portals to the bases inside the Battle for Mount Hyjal instance. Before Patch 3.0.2, the officers' lounges in Stormwind and Orgrimmar were also behind white portals.
Purple Instance Portal: all instances in Outland, Northrend, the Caverns of Time, the Magisters' Terrace, those introduced with Cataclysm and Mists of Pandaria (plus the Deadmines and Shadowfang Keep) are of this color. Indicates that the instance has a Heroic mode available.
Skull with Purple Instance Portal: Notifies the player that the 5-player instance is on Heroic mode.
Skull with Green Instance Portal: Notifies the player that the raid instance is on Heroic mode. Only available to Trial of the Crusader, Icecrown Citadel, Ruby Sanctum, Cataclysm and Mist of Pandaria raids.