Mike, that's a hugely interesting post re the yellow diabolo. What's your gut feeling re the 21st being blue or yellow?
You quite rightly makes mention of deception, and this may be the case here too? Also, the Germans had spies everywhere, just like us, and it's possible they obtained a copy of the early WO (yellow diabolo) communication and it was never updated with blue? So many variables, but all guesses. However, surely the diabolos must have been accurately recorded here in the UK somewhere? As a last resort, do you know if any of the 21st ATB RTR units still have an accociation? We could ask any surviving veterans?