EDH Colors

[Subiskier]

I have an EDH deck where {glissa the traitor} is general. Am I allowed to play {crypt ghast} in it since the extort cost has a plains in it?

[Agrus Kos, Enforcer of Truth]

I don't know of a rule that would stop you...

[Keyeto]

I have an EDH deck where {glissa the traitor} is general. Am I allowed to play {crypt ghast} in it since the extort cost has a plains in it?

You are allowed. EDH cares about color identity, and since the white cost is in the reminder text for Extort, the following rules applies:

903.4b: Reminder text is ignored when determining a card's color identity. See rule 207.2.

Your Ghast is good to go!