Condition green was a pre-arranged code phrase used by landing parties of the USS Enterprise when communicating under duress. What it indicated was that while the party was in trouble, the ranking officer aboard ship was not to intervene or send anyone to help, as this would only endanger more people. It did not prevent that officer from preparing for possible future actions. Neither did it prevent that officer from using considered judgment to exploit an opportunity that aided the party without violating mission parameters. (TOS: "Bread and Circuses")
This, and other known code phrases like chess problem were applicable only under certain circumstances and rarely used. Presumably, such codes were part of a "play book" devised by Captain Kirk and his officers (citation needed • edit) to cover a wide range of scenarios. (TOS: "Whom Gods Destroy")
In "A Piece of Reaction" Commander Riker, under duress from the Sigma Iotians, told the computer on the Holodeck to cancel a self-destruct sequence, adding "condition green" as his authorization code, prompting the computer to respond "Condition green acknowledged." This convinced his captors that the self-destruct sequence had been aborted, when in fact it was actually still running.