Why red colour is used as danger signal
Where human intent is involved, red is used to indicate (potential) dangers because it is relatively easier to see or distinguish compared to other colors. Red light, which is what our eye detects as the color red, is scattered the least by air molecules. Red light has the largest wavelength and so is more easily perceptible even through harsh weather, low lighting or overcast skies, and across distance. This means that even if it's very rainy or dark out, you'll have an easier time seeing a red object than one of another color to indicate danger.
Humans have also evolved to associate the color red as danger, though the origins of this are difficult to determine. Many plants and animals in nature use bright coloring as a means of warding off predators by either intimidation or indicating a poisonous quality. For example, poison dart frogs come in a variety of colors, all quite bold. This sends a signal to other animals that they are not good for eating. Many poisonous berries are of a bright red color, and this association lead to the myth that tomato fruits were poisonous.
Based on cultural associations with color, red may not be the color of choice for signalling danger. Alternately, the color red may have meanings that seem to contrast with the idea of danger. In the United States, red indicates both love and danger.