Rainbow Code
The Rainbow Codes were a a series of code names used to disguise the nature of various British military research projects from after the Second World War until 1958 when they were replaced by an alphanumeric code system.
The Ministry of Supply (MoS) initiated the idea since during the war some German secret projects could be inferred from their code names.
Each code name was constructed from a colour of the rainbow plus a noun eg "Blue" + "Steel" gave Blue Steel, a British nuclear weapon, "Green" + "mace" = Green Mace, an anti-aircraft gun.
The names where dropped with the end of the MoS, and their functions were handed over to the Ministry of Defence which started using codes built of two letters and three numbers eg BL755