If you are trying to match local color in your painting think about the following:
Is the local color darker or lighter than the color I have mixed? I like to hold my brush out in front of the object I'm painting to see if the value is correct.
Is it more neutral, or more intense than my mixed color? This one is tough because our personal style and vision effects the way we see color intensity. I tend to see more color saturation than some other painters do. It is very hard for me to be tonal and much of the tonal paintings I see seem dull and bland to me.
Is it colder or warmer? I am referring to color temperature here not air temperature. Looking at the color wheel, is the green you see more red or more blue? The red, yellow orange tones will be warmer than the cool bluish greens.
Consider these questions when you are mixing your paints, especially when you work from life.