Complex sentence: The Rainbow Codes were 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.
Split into simpler sentences: The Rainbow Codes were a series of code names used to disguise the nature of various British military research projects. They were mainly used from after the Second World War until 1958, when they were replaced by an alphanumeric code system.