The original yellow ribbon, the one that proclaimed a young woman the sweetheart of a US cavalryman, came about because of the yellow stripe that ran down the outside of the cavalryman's pants.

If I remember correctly, yellow was cavalry, red was artillery and blue was infantry.

A variant of this song was still sung in Army basic training units as late as 1971, the differences being the young woman wore a "trouser blouser," the elastic that kept the cuffs of your fatigues neat without having to tuck them into your boots, and she wore it for "Joe Trainee" who was far, far away.