Originally Father Christmas wore green rather than red, supposedly to symbolise the coming of spring. Over the years, Father Christmas and Santa Claus have become one, which means they both wear red. Thomas Nast, a popular American cartoonist, drew a picture of Santa for Harper's Weekly magazine in 1862. In 1931 Coca-Cola ads began appearing in popular magazines. Archie Lee, the D'Arcy Advertising Agency executive working with The Coca-Cola Company, wanted the campaign to ensure people continued to drink Coke during the winter months, as the drink was associated with warm, summer days. The red suit was a convenient feature of the character as it matched the advertising colours of the company.
Maybe you've heard that blood is blue in our veins because, when headed back to the lungs, it lacks oxygen. But this is wrong; human blood is never blue. The bluish color of veins is only an optical illusion. Blue light does not penetrate as far into tissue as red light.
Blood is always red. Blood that has been oxygenated (mostly flowing through the arteries) is bright red and blood that has lost its oxygen (mostly flowing through the veins) is dark red. Anyone who has donated blood or had their blood drawn by a nurse can attest that deoxygenated blood is dark red and not blue.
Thanks to its long wavelength, red is one of the most visible colors in the color spectrum (second only to yellow). Its it is able to travel the longest distance through the fog, rain, etc. to instantly grab people's attention which is why it's often used to warn people of impending danger. Think: stop signs, sirens, fire engines, and red traffic lights.