There already seem to be enough good answers, and I would probably go with red myself, but @maxathousand asked me post a comment of mine as an answer.
While red is generally seen as "danger" or "stop", that is fairly recent and may be culture specific (for instance, the Chinese consider red to be auspicious (which once led to me wearing some very gaudy "auspicious underpants" for Lunar New Year when I was working in Shanghai)).
If you really want to speak directly to the lizard hind-brain, then use black & yellow - either as the icon colo(u)r, or as a border to delineate it.
See, for instance, Why does black and yellow indicate danger? on our S.E. biology site, which has some interesting explanations as to why we, and other creatures, fear this combination.
Or, just think of wasps & bees, maybe you know of some snakes & frogs which are black & yellow?
In industry, I have often black & yellow floor markings delineating potentially dangerous areas, black & yellow tape on drums of hazardous materials, etc. I would not be surprised if the same login holds true for "Police line! Do not pass!" tape.
So, perhaps if if would blend well with your chosen colo(u)r scheme, this could be an option?