The Rainbow Codes were...
Maybe practice varied according to department because RV Jones mentions the choice of meaningless codenames in his wartime memoirs.
Our friend The Wikipedia also says:
In the
Second World War, code names common to the
Allies referring to nations, cities, geographical features, military units, military operations, diplomatic meetings, places, and individual persons were agreed upon adapting pre-war naming procedures in use by the governments concerned. Random lists of code names were issued to users in alphabetical blocks of ten words and were selected as required. Capricious selection from the available allocation could result in clever meanings and result in an
aptronym or
backronym although policy was to select words that had no obviously deducible connection with what they were supposed to be concealing.
It is also possible, though, that the name resin just referred to the material used for some lamps.