If an area of green is adjacent to an area of white and an area of black, it will look darker next to the white and lighter next to the black. Think about how you might use this in your work. Also look at the light that is falling on your subject. Different types of light can make a dramatic difference in the appearance of colors. Incandescent lights are typically yellow or orange in tone while flourescents are more green.
Sunlight is the whitest light.
The brain tends to correct for these variations and we see most light as white unless two types of lights are right next to each other. Photographers, however, are well aware of this phenomenon as film must be specifically formulated for the type of light available or photographs will show a distinct color cast. Other types of artists would be well advised to study light as well as they may be able to use the effect in their work. For example, an indoor scene may be rendered in more yellow tones to create the feel of incandescent lighting. Take Your Time Looking is a fast process - seeing is not.