The initial and previous flag (that “by default red triangle one”) was broken, so I decided to look around on the web. Without surprise, I took a Purple Sky Neon Stripe (L) one : these flags are just great (I do not list all there advantages, some others have already done it), and I chose – of course – a large Neon one. This color looked to me as the best one when talking about being as visible as possible. And that color just fits very well my lemon helmet 
As already stated by others, do not forget the “C” clip, and also lightly burn both ends of the streamers, in order to stop the fraying.
