Robert Comsol wrote:
My first thought was that the pairing is deliberately redundant, to aid any individuals with color blindness, or possibly individuals from species with monochromatic vision.
You already provided the answer: Back in the 1960's during TOS' original run there were still many households belonging to a "species with monochromatic" black & white only television sets.
True, but then the screen size and resolution of 1960s sets wouldn't have rendered the assignment patch as easily visible and distinguishable. Even on my big-screen TV the symbols aren't always easily seen, especially with the sparkly background they're on.