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Aboard Starfleet vessels and outposts, a series of color-coded alert signals were issued, usually by the ship's computer or internal communications systems.
The Earth Starfleet began without an alert status system (signal) until one was suggested by Lieutenant Malcolm Reed aboard the prototype Enterprise, an early starship that had a larger crew and a greater need for reaction time during crisis situations.
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.
In this status, a ship's deflector shields were brought to full power, but the weapons systems remained off-line. Depending on the situation, the captain could order a reduction in the closing speed between itself and any potential incoming threats.
A yellow alert could 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. If the situation escalated, the commanding officer or the main computer would order a red alert.
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.
One stage above a yellow alert, this status had weapons systems and shields brought to full power, essentially preparing the ship for combat.
Red alert could also be 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, an evacuation order, and a viral medical emergency, among others.
A red alert could either be activated manually by the commanding officer or the officer in charge, or automatically, as when a ship was attacked, entered a dangerous area, et cetera.
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.
The USS Defiant went to blue alert when engaging its Romulan cloaking device in 2371.
On a vessel with landing capabilities, such as the Intrepid-class, blue alert was called as an indication that the ship was preparing either to land or to take off; the crew would then need to report to code blue stations, in order to aid in the landing/liftoff procedures.
Blue alert was also used when a Prometheus-class starship activated its multi-vector assault mode.
Black alert, otherwise known as condition black, was an alert signal used on the USS Discovery and the USS Glenn in 2256 to signify that the ship was operating its experimental spore drive.
When in gray mode, the starship ran on reserve power, cutting power to non-essential decks and ship systems. It allowed for maximum power conservation, budgeting a starship's energy resources during a time of crisis, while still maintaining an operating status. Gray mode could be invoked during a massive fuel shortage, or when the tactical situation required a reduction in discretionary energy usage.