I am skeptical of at least part of this story. I grew up in California and I can tell you as a primary historical source that bakeries were routinely using pink boxes for cakes and pies well before the late 70s/early 80s.
It’s probably true that the boxes were used by doughnut shops because they were cheap, but I don’t think they were cheap because there was some leftover pink cardstock that a doughnut shop owner was wiling to settle for. I think pink bakery boxes were just easy to come by.
