It’s an open question. One physicist’s answer is that red colours are scattered least by fog or smoke, and hence can be seen from furthest away. Another physicist’s answer is that the receptors for red colours in the eye are clustered in the area near the centre where the sharpest images are formed. A biologist’s answer is that nature uses red as a warning colour because it stands out most vividly against a green background. Other answers are that we associate it with danger because it is the colour of fire and blood (source).