Aboard Starfleet vessels and outposts, a series of color-coded, and also generally named, alert signals were issued, usually by the ship's computer or internal communications systems.
Condition Green refers to the normal operating condition (cruise mode) of a starship. In this state the ships shields are down, its weapons are on cold-standby and torpedoes are not loaded.
Yellow Alert or Condition Yellow is the second highest alert signal status on Starfleet vessels, one stage below red alert and one stage above a condition green. It designates a ship-wide state of increased preparedness for possible crisis situations.
In this status, a ship's defense fields are energized, and/or the deflector shields are brought to full power, however, the weapons systems remain off-line, torpedos are loaded and placed on standby. Depending on the situation, the captain may order a reduction in the closing speed between itself and any potential incoming threats. The ship's power output is increased as shields and weapons charge.
A yellow alert may either be activated manually by the commanding officer in preparation for a tactical or environmentally hazardous situation, or automatically upon detection of unknown or hostile spacecraft, or upon detection of certain system malfunctions. The Captain, First Officer, or OIC (officer in command) are the only persons who can order a change to this alert status.
Red Alert (also Condition Red or Code Red) is the highest alert signal status on Starfleet vessels, one stage above a yellow alert. In this status, weapons systems and shields are brought to full power, as the ship prepares for combat. All senior officers are summoned to the bridge and security teams/marine guards are stationed at all critical locations to include, the Bridge, Marine CIC, Engineering, Operations Center and Security Offices, Armories, etc. Red alert is also ordered during various other emergency situations, such as a ship being boarded, radiation exposure, a security alert, massive systems failure, an imminent warp core breach, and an evacuation order among others. During a red alert, lights are dimmed, alarms sound, and computer terminals show constant red graphics.
A red alert may either be activated manually by the Commanding Officer, Executive Officer, or the officer in charge or automatically (when a ship is attacked, enters a dangerous area, etc). The raising of shields automatically triggers a red alert. The Captain, First Officer, or OIC (officer in command) are the only persons who can order a change to this alert status.
Intruder Alert: is usually an automated response and triggers a Red Alert if internal sensors detect a creature or individual which boards the ship without authorization. Initially, the alarm only sounds on the bridge where the computer/staff will attempt to identify the intruder. With the Captain/OIC’s permission, a ship-wide Intruder Alert can be issued, in which case all hands are armed with type two phasers and will attempt to apprehend anyone who appears dangerous or who matches the description given.
Blue Alert (also Code Blue or Condition Blue) is an alert signal status on Starfleet vessels and outposts used in exceptional situations which range from environmental hazards to the crew, to docking and separation maneuvers, landing protocols for ships with the capability, and silent running operations.
In the event of an imminent environmental systems failure or disruption, a Blue Alert is called in order to help affected personnel escape or safeguard their lives.
On a vessel with landing capabilities, a Blue Alert is used as an indication that the ship is preparing either to land or to take off; the crew would report to code blue stations, in order to aid in the landing/liftoff procedures.
During silent running operations, ships outfitted will activate them. Certain non-critical systems are deactivated including most recreational systems and active sensors. Long-range sensors are available, but short-ranged active sensors are not recommended (active sensors can cause a "ping" which alerts another vessel that it is being scanned).