Hi-Viz Jackets
As an "arm waver" we always wear one (for obvious reasons). In our little group of friends that do this everyone except my sister and I wear yellow. We wear orange.
This stemmed from our Air Cadet days when we all wore yellow and to make life easier for people those who want to know who was in charge we wore orange so they could be directed to either of us without problem.
It can get confusing at "our" fly-ins when all sorts of people are wearing a yellow H-V Vest....
And if the sun is low and behind you pilots struggle to see us... whatever the colour and size of bats!
Slightly off topic....
It surprises me that anyone can arm wave with no real training. We won't let anyone unless we've trained them up and are happy that they be safe and sound. If anyone becomes dangerous then off they come! (and refreshed if required)
Oh in case you are wondering I have been doing this arm waving lark in my spare time for 15+ years at various events (including PFA Rallies) and my sister has about 10 years experience.