I had thought this subject had been previously raised, however, I could not locate it.

Came across the following in Dr. McPheeters diary, I Acted from Principle, Pitcock & Gurley, U of Arkansas Press,
2002, p. 343, Note 23

"Yellow was the color adopted by the Confederate medical corps. Yellow flags were typically flown over hospitals
to denote their noncombat status."

George Martin