Nothing describes the use of secret inks better than a real case study.
What follows is the case of WW1 spy, George Vaux Bacon. Although he was an American, George spied for the Germans in 1916. He received his orders from his handlers, Albert O. Sander and Charles Wunnenberg.
He was to travel to Rotterdam in Holland, a country that was, in fact, neutral in WW1.and he was to be paid the princely sum in those days of 25 British Pounds a week.
Bacon was very concerned about how he would communicate with his new employers to whom he expressed his fears. Sander calmed him down and showed him how it would be done. He asked Bacon if he had a pair of black woollen socks and bacon dutifully handed a pair over. He watched in amazement as Sander produced a toothpaste tube from which he squeezed a dark brown paste and spread it over the tops of the socks. All Bacon had to do was to soak the socks in water which would release the brown paste from the tube and the resultant mixture produced the invisible ink solution.
And that was in WW1, imagine what they were doing by WW2. There will be more case studies on this topic to follow.