A phrase commonly used
in World of Warcraft
to tell players
to avoid spells (like poison clouds, rain of fire, tail sweep and similar) that often deals
massive damage to players
can cause a wipe
This phrase apply to all kinds of harmful spells, even if they're not coloured green or coloured at all.
The early boss Grobbulus
in the raid-instance
Naxxramas in WoW
have the ability to put out green
poison clouds under him. If the raidmembers stand in it they
will die because of the damage.
The raidleader have been
explaining the tactics and ends with: "And don't stand in green