Scientifically, red is the color that can be seen farthest and soonest, even when there is not much light. If I remember rightly, that's because it is the color with the longest wavelength. So choosing red to warn is logical for traffic signs and warning symbols, such as the red arrow.
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'Mercy!' cried Gandalf. 'If the giving of information is to be the cure of your inquisitiveness, I shall spend all the rest of my days in answering you. What more do you want to know?' 'The whole history of Middle-earth.. .'
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