Well, as I've already mentioned, many keyboards had either a red ATTN key (IBM 2741) or a red BREAK key (ASR 33 teletype), and so the key used to interrupt a running program, being perceived as dangerous, is red to warn people to not push it at the wrong time by accident.
On the basis of precedent and tradition, therefore, I am an advocate of a red Pause/Break key instead of a red Esc key. I know that this makes me a voice crying in the wilderness here.