HuwG - yes the hackle is different to the cockade - it's more like a shaving brush, and in Napoleonic times it was white over red for everybody but riflemen (green) and grenadiers (white), as far as I remember.

I've heard the song as the 'blue' cockade, but no-one that I know of wore a blue cockade either, at least not in our armed forces. The cockade was a national thing and the British/UK one is black.

So I tend to think of the white/blue cockade as a local mark for a new recruit, or he's enlisted in a militia or volunteer unit.