MI6
Sir John said that he keeps a special coloured pen in his desk for official correspondence and to inspire junior agents with the history of the service.
MI6 bosses have been known colloquially as the "green ink brigade" since the days of Sir George Mansfield Smith-Cumming, the former Royal Navy officer who established the Secret Intelligence Service (SIS), also known as MI6, to gather intelligence on the Germans before the First World War.
Had never heard of this before. Interesting piece of history.