Like notaries, witnesses aren't generally required in order for a contract to be legally enforceable. However, each state may require special kinds of contracts (e.g., real estate deals, wills, and marriage agreements) to be witnessed by a third party in order to be binding.
McDonald's, apparently, use red and yellow because red=fast and yellow=hunger (hence fast food!). Remember also that meaning is what we create. It does not exist in the color itself and individual meanings may or may not exist in different cultures and individuals.
Red gives the impression of seriousness and dignity, represents heat, fire and rage, it is known to escalate the body's metabolism. Red can also signify passion and love. Red promotes excitement and action. It is a bold color that signifies danger, which is why it's used on stop signs.