Red catches people's attention, and is often used either in a negative way to indicate danger and emergency, or in a positive way in advertising to gain more viewers, or in nature, as a ripe fruit announces its readiness with its red color. Several studies have indicated that red carries the strongest reaction of all the colors, with the level of reaction decreasing gradually with orange, yellow, and white, respectively. Because of this, scientists have repeatedly recommended red for warning signals, labels, and signs. Because of these recommendations, red has seen widespread use as a danger signal, in stop signs, to warn people of extreme heat or flammability, and even to signal warnings in sports such as soccer.
Red is one of the most common colors used on national flags throughout the world. On these flags it carries the same meanings which it does anywhere else: the blood, sacrifice, and courage of those who defended their country, the sun and the hope and warmth it brings, and the sacrifice of Christ's blood (in some historically Christian nations) are a few examples. Red is the color of the flags of several countries which once belonged to the former British Empire.