Choosing colors for a painting should be fairly easy. We all know that the sky is blue, the grass is green, skin is pink and eyeballs are white.
However, most of the time the sky is more gray-blue than blue, especially close to the horizon it is often light gray. The grass is often more dirty, brownish green and eyeballs can be very dark, depending on the lighting and shade.
It is therefore a constant struggle for a painter to make all colors seem life-like and natural.
Another problem is that the human eye (and brain) perceives an object's color not based on its actual color, but on how it compares to the surrounding colors.
Checkout below some baffling examples of optical illusion.
Click on the pics for large examples and explanations.
Because of those possible pitfalls I frequently use a "color checker" tool. More about that under "Oils".