The JMC spaceship Red Dwarf and its shuttle-craft such as Starbug had a colour-coded alert system. When the ship or crew were under threat, the ship computer Holly or a number of other automated voices - including sirens if needed - would warn the crew and tell them which type and colour of alert it was.
The most severe type of alert was a Red Alert, which meant that the ship was in imminent danger of failure, catastrophe or was being attacked. A Blue Alert meant that there was a possibility of danger, but the threat had not yet been properly identified or ascertained. The least severe level of alert was a Yellow Alert, which meant that the ship was not in any danger, but was receiving a distress signal from another vessel.
Red Dwarf has been seen under Red Alert on several occasions, for example when the Cadmium II radiation leak occurred (as seen in Me² and Skipper). Other examples include in "Queeg" (when hit by a meteor); "Marooned" (when on a collision course with a Black Hole); "Bodyswap" (when rewiring by a deranged skutter initiated the Self-Destruct System); "Demons & Angels" (when the Engine Core had been overloaded); "Beyond a Joke" (when under attack from the SS Centauri); and in "The Beginning" (when under attack from the Simulant Death Ship).
When the Dwarfers were traveling to an ocean moon in Starbug for a "fishing holiday", Holly detected a massive spatial distortion in front of the 'bug. Holly blurted out to the others that it was a "Purple Alert", and the others did not understand what this meant. Holly said it was not as bad as a Red Alert, but a bit worse than a Blue Alert, and she didn't want to say "Mauve Alert". The distortion turned out to be Ace Rimmer's ship Wildfire dimension-hopping, and it collided with the 'bug causing a crash landing into the ocean moon. ("Dimension Jump", Series IV)
When the tractor beam of the Legion Station came close to Starbug and looked suspiciously like a missile, Arnold Rimmer ordered Kryten to go to Blue Alert to remind them that they should all be on their toes, even though Dave Lister said they already were and were all present. Rimmer was glad 'for a bit of professionalism' as the Blue Alert box started flashing. As Rimmer got more worried, he ordered Kryten to go to Red Alert. Kryten asked him if he was sure, since it did mean changing the bulb. ("Legion", Series VI)
Rimmer once performed a drill aboard Starbug, and pretending it was a Red Alert he ran into the bunkroom where Lister and Cat were sleeping shouting "scramble". Lister said that it would great - with bacon and beans. Lister and the Cat finally joined Rimmer and Kryten some hours later after having four rounds of toast. Ironically, the 'bug then came under a real attack from an Enforcement Orb. ("Emohawk", Series VI)
When the nanobots recreated Red Dwarf, Starbug got sucked into the massively enlarged ventilation ducts. Kryten suggested that they be bold and decisive by moving the alert colour up from Blue to Red. Cat suggested that they go all the way up to "Brown Alert". When Kryten said that Brown Alert didn't exist, Cat said that Kryten would soon agree that that it did, and that he had been warned. ("Back in the Red Part I", Series VIII)
When Red Dwarf came across an unmanned craft emitting a distress signal, Red Dwarf was placed under Yellow Alert. Rimmer said that it was his most favourite type of alert, because that meant he wasn't personally in danger. However, Lister and Cat had unwittingly downloaded an SOS virus from the unmanned craft, causing chaos aboard Red Dwarf and eventually a greater degree of self awareness among the various AIs of the ship. ("Mechocracy", Series XII)
When Red Dwarf was hit by the Quantum Anomaly, causing a ship-quake, Cat said to Lister that it wasn't serious unless the calm voice lady starts to talk. A woman's voice then came over the loudspeaker, asking them to keep calm. They then started panicking. ("Skipper", Series XII)
When Rimmer used the Quantum Skipper to explore different dimensions, he found himself in a version of Red Dwarf where the Cadmium II radiation leak had not occurred, and Holly was still the ship computer. When the disaster then began to unfold, Holly did away with protocol and set off sirens and shouted "Radiation leak alert! Radiation leak alert! All crew should run around screaming!" ("Skipper", Series XII)
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