The color red pretty much universally signals DANGER! That could be for a number of reasons.

1. - fire is red, fire is dangerous, therefore red means danger
2. - blood is red, and if there is a lot of it about it usually means something bad, i.e. dangerous has happened, therefore blood=red=danger
3. - people who are angry tend to do dangerous stuff, but also get red in the face while doing so - though that's due to blood too
4. - red is the sort of colour which gets noticed in a hurry. You want people to take note of danger quickly so you use red for your signs.