Nguyen Cao Ky leapt from the Air Force of South Vietnam to become a politician. Early example of French (he was French trained) then American intervention, choosing locals in a way which didn't necessariliy result from the closest research. In consequence, US ended up shackled to some dodgy 'friends', who in turn did the overall cause no good at all, just as they have more recently.
As Jim says, he used a purple scarf as a form of trademark.
just a stab in the dark here, but i train in commando krav maga which is in use with isralei special forces at the moment, and i have heard many high up ranking people in the system refer to a purple scarf. they use it to describe the result of drilling various choke and strangle techniques for hours at a time. basicly the hideous friction burns you tend to recieve when having these techniques perfomed on you with full force over and over again.
obviously it may not be this, but it depends how he was told this.
the fact its 'the' or 'a' suggests its a nickname.
and if he was told this by a soldier means its probably not a referance to a vietnamees pilot of the 60s.