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.
The warmth of red, yellow or orange can represent excitement or even anger. Warm colors convey emotions from simple optimism to strong violence. The neutrals of black and brown also carry warm attributes. In nature, warm colors represent change as in the changing of the seasons or the eruption of a volcano.