U.S. Army branch colors are not a 'Hollywood invention'.

Whether or not calvary troops wore neckerchiefs may be debatable, but I assure you that yellow is indeed associated with cavalry, scarlett with artillery, and sky blue with infantry. Matter o' fact, we can add green for the military police. (I have the damn neckerchief to prove it!)

So, while this may have no bearing on the original 'handkerchief' question, as was so tactlessly pointed out, it DOES bear on the 'Yellow ribbon' post.

Cheers!