Warning

Red catches people's attention, and can be 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.[48] 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.[49] Because of this, red is often used to catch people's attention in a variety of situations (see: penalty card).[50]

The port, or left, side of a sea-going vessel carries a red navigation light, to warn other vessels approaching from that side to change course to avoid a collision.