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 is associated with high energy and power. It is the color our eyes are drawn to first in a room, so a little can go a long way. Red signals courage, ambition and strength. It promotes alertness and speed, and connects us to our physical self.