This is a question of local versus actual color. Different objects like green apples and oranges have a certain local (such as green or orange), but you never paint them with green paint or orange paint. Why is this? It is because that when light falls on a green apple or an orange, it changes color. If the light is cold, for example from the blue light of the sky, the green apple becomes a blue green, and the orange becomes grayer. If the light is warm red, for example at sunset, the green apple gets grayer, and the orange gets more intense and redder. So the actual color you see is very different from the local color. This is why you can never give a formula for painting any object (such as skin), because the color is determined entirely by the color of the light falling on the object. If someone gives you a formula for painting skin, ignore it. This is why you it is essential you learn how to see color (Observation 2), and understand what happens to the color of objects as it falls across forms (Form 3).