The colors that we see everyday are "local" colors. The grass is green, a lemon is yellow. Pretty easy. Local color is the color of an object when viewed at a distance under normal lighting conditions. It is what we are taught as preschoolers. But this can be deceptive. Is the lemon all yellow? Are there other colors there when we look close?
In real life, there are several colors that may be perceived, green, orange, blue, red, etc.. This all depends on the lighting conditions. These colors are call "reflected" colors. Reflected color is the color of an object under the influence of atmosphere, light and shadow. All the real life surroundings of the object affect the color that we see.
The job of an artist is to render those differences in colors and values to the viewer to convey a realistic object with a life of it's own. The more that an artist paints, the more they will see.
I've seen a lot of how-to paint tutorials on the internet giving color formulas for different objects. Artists need to learn to see line, shapes, values and color. When someone is given a formula, there is no need to see or learn how to mix colors.
Painting is all about comparison. Everything is compared to the first stroke that we lay down on the canvas. The next stroke, is it lighter, darker, warmer or cooler? Does one color have a little more green or red in it than the next? There is constant comparison between one stroke or object and another one. At times, a color can be hard to define, but look at it.