Blue alert (also code blue or condition blue) was an alert signal status on Starfleet vessels and outposts which was called for in exceptional situations, including, but not limited to, environmental hazards to the crew, main power failure, docking and separation maneuvers, and landing protocols, for ships with the capability.
A green alert, condition green, green light, or green status was an alert signal and status report used on Earth and later Starfleet vessels. It had a wide variety of uses depending context, but generally was the regular status of vessels when not under any other alert status. Green alert also signified that ship systems were operating correctly.
A yellow alert or condition yellow was the second highest alert signal status on Starfleet vessels and starbases, one stage below red alert. It designated a ship-wide state of increased preparedness for possible crisis situations.
Red alert, also known as condition red or code red, was the highest alert signal status on Starfleet vessels and starbases. It was usually triggered when entering a combat situation, or in the case of a critical systems failure.
IIntruder alert was an alert status on Federation starships and space stations. It could be declared at any time, by computer or any crewmember upon detecting an intruder on board. It could also be activated automatically based on predefined criteria, such as entering a restricted area. The alert was announced by audio and crewmembers were expected to take appropriate action. In 2266, a security alert 3 announced a possible intruder aboard a vessel. (TOS: “Dagger of the Mind“) Intruder alert was also known as “general quarters security condition three”. (TOS: “The Man Trap“)