After the switch to the cypress green lid, apparently the DofE arrived at Ashford resplendent in his new Int Corps beret, to be met by the chief slimer. "Sorry about the new beret sir" quoth he, "I don't know what ee-jit chose the colour!"
I would imagine the int corps adopted it as their beret colour from greek mythology as it was sacred to Hades, the king of the underworld. He was also supposed to have a magical helm that rendered the wearer invisible which he often lent out on one occasion to Perseus so he could kill Medusa, hence the association with sneaky beaky shit. Ok I dont really know but that sounds good.
It was a magical day that the AGC (SPS) also started to wear the Cypress Green beret, especially at Thiepval Bks where many of them dulled it down with boot polish, unless you were really close up they looked like they were badged Int Corps. I think one of the words used to discribe them was 'Throbbers'.
Not Int Corps but a bit of a spotter and daring to be sensible in this post
I believe it was to do with the original Int Officers who wore their own regimental accroutments but also wore green tabs on their lapels. Why Cypress green ? I'll go with the mythology just look at the SRR cap badge