I guess they thought that was clever.
IAE, the Left did not consider yellow or red to be racist terms back then.
When I was in the RVN we never called them yellow men, we always called them the NVA, VC, or little Brown Men. (and they weren’t Mexicans)
Contrary to popular belief Asians are not yellow, they are a light sugar brown color, and even that’s not true for all of them.
Of course we called them a lot of other things too, but to the best of my recollection never yellow.