According to Bug Guide, these brilliantly pink box elder bugs are ones that have just transformed from the nymph to the adult stage. I also wonder if they briefly become this brilliant pink, when transforming from one stage to the next as nymphs, because I've occasionally seen small nymphs with this rosy glow. The colour is striking and really stands out and at first, I was not at all sure that I was really looking at a box elder bug, despite it being amidst an aggregation of these bugs.