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.
Yellow-is the most visible color and is the first color the human eye notices! Yellow, the color nearest to light leaves a warm and satisfying impression, lively and stimulating and in many cultures symbolizes deity. Dark yellow can be oppressive while light yellow is breezy.