What is the color of flesh? What is the color of an apple? What is the color of grass?
What is the color of the sky? The answer is simple: variations of red, yellow, blue and white. (If you use the cool/warm method, just add a cool and warm of each color: cool red leans towards violet and a warm red that leans towards orange; warm blue leans towards red, cool blue leans towards green; warm yellow leans towards orange, cool yellow leans towards green.)
Nothing is just one color, due to shapes, texture, background, lighting and shadows. Everything you see is variations of white, red, blue and yellow. And yes many times you will need some sort of black. However pigment black changes the color properties of many colors, especially yellow, so it is best to tone a color down by using it’s opposite color or analogous color instead of using black. You can create blacks by mixing a dark brown and a dark blue, red or green (depending on what shade the black color is).
Mix flesh colors by making color portrait sketches in a notebook. You will notice a person’s colors vary each time you paint them, whether it is from a sun tan, different lighting or different color clothing or background. This applies to almost every subject you can imagine. Play around with your pigments! The more you teach yourself how to mix colors by memorization of basic formulas, the less time it takes to paint. I know oil painters who will sit there for hours just premixing colors for each object in their paintings. They will first mix the lightest light, then the darkest dark, then mix the mid tone, then mix the in between colors (usually totaling nine tonal mixtures). It’s a waste of time and paint!
If you know your palette well, you will not have to premix all those colors, you can just look at the subject and know exactly what colors you need to do the job and do it! You never need an elaborate road map to any destination if you already know the address, where you are, and which streets to take by memory and experience! You just drive there. I can teach you how to drive and show you the streets of every destination you wish, and to take the scenic, side streets and quickest routes. I can’t warn you about bad squirrel decisions, detours, emergency vehicles or predict the weather. But I can tell you how to deal with it when those situations arise. In the end, it is up to you to memorize and apply the information given to you.