Aboard a Starship/Starbase there are several different states of alert that the ship/facility may be placed in depending on its status. A breakdown of these alert conditions are below.
A Blue Alert; also Code Blue or Condition Blue; is an alert signal status on Starfleet starships and outposts which is called for in exceptional situations, ranging from environmental hazards to the crew, to docking and separation manoeuvres. Blue alert is also called for landing protocols and when cloaked, for ships with the capability.
In the event of an imminent environmental systems failure or disruption, blue alert is automatic by the computer in order to help affected personnel escape or safeguard their lives. Blue alert does not supersede red alert but it can be called over yellow. On a vessel with separation capabilities, such as the Galaxy Class, blue alert is called as an indication that the ship is preparing either to separate or reconnect; the crew would then need to report to code blue stations, in order to aid in the these procedures.
A green alert; also condition green or code green; is the lowest alert signal status on Starfleet vessels. A vessel will operate at green alert unless faced with any hazardous situations. Green is the typical status during normal shipboard operations.
A yellow alert; also condition yellow or code yellow; is the second highest alert signal status on Starfleet vessels, one stage below red alert. It designates a ship-wide state of increased preparedness for possible crisis situations. In this status, shields and deflectors are brought to full power; however the weapons systems remain off line.
A yellow alert may either be activated manually by any member of the crew 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. If the situation escalates, a crewmember or the main computer will order a red alert. During yellow alert the on duty crew will prepare for possible combat or hazardous conditions. Any of the senior staff that are not on duty will be contacted and ordered to attend their positions.
A 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. 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.
The red alert may either be activated manually by any member of the crew or automatically when a ship receives damage or upon detection of certain system malfunctions. During a red alert, lights are dimmed (to allow better viewing of the LCARs system), alarms blare, and computer terminals show constant red graphics. Upon activation of the red alert all crewmembers are required to attend their designated combat stations. All civilians will move to designated civilian 'safe zones'.
The auto-destruct system; also known as self-destruct or destruct sequence; is a starship system that allows the total destruction of the vessel. This is typically activated as a last resort, usually to prevent a ship from falling into enemy hands.
On Starfleet vessels, the activation sequence varies from class to class, but usually requires authorization from the Captain and/or senior officers, with the use of command authorization codes in most cases. Once the countdown is activated, the computer gives audio warnings, usually with an interval of 10 seconds until the last 10 seconds in which case in counts down in intervals of a second, and displays graphics on view screens. The countdown can be rendered silent by the senior officer that initiated the sequence.