Reading SAS stories, I often come across the nickname "green slime" for the Army Intelligence Corps. I'd hazily thought it was a squaddie comeback at the alien, slightly threatening nature of the people who know more than they'll ever tell you.
It's a bit simpler than that. The Corps beret is a bright green, and so when massed on parade the soldiers will seem to be a moving, verdant carpet.
The kit was devised by its Colonel-in-Chief, the Duke of Edinburgh. The CIC came down to Regimental Headquarters for an inspection shortly after the new outfit had been issued, and asked the Sergeant-Major what he thought of it.
"Bloody horrible, Sir."
"Did you know that I designed the uniform myself?"
"Well then, we've both made a mistake, haven't we, Sir?"