Color is a key component in an illustration in aiding the viewer in establishing the correct emotion. A very simple example is the use of complementary colors like red and green. This combination in most situations in the American culture makes people think of Christmas. While looking at the color green alone the viewer may assume growth, fresh, go or safe, it does also have some negative associations like greed and jealousy. It is amazing how adding the complementary color of green to red has created this holiday brand color.
By itself color red has other emotional effects on people like the association of heat, fire, power, it has also been associated with a signal to stop. The color red is even used to represent love and passion.
Pink is another color that has been a way to establish femininity. Most children wearing pink clothing are assumed to be female. Also consider how the popular retailer Victoria Secret has taken the color pink and branded their store with this color. Even the Breast Cancer Awareness has adopted this color for their movement.
Some color combinations even can effect how the piece is visually viewed. For example Warm color combinations have a tendency to appear to move in the direction of the observers and seem to be in a closer proximity. To push away from the viewer one could chose to use a cool color combination, which would will aid in the scene pulling back from the viewer. In other words, warm colors will appear to be larger than cool colors.
One key thing to keep in mind about color is a viewer may respond differently to color depending on their culture and background. While it may seem that response to color could be a natural response, it is really based on conditioned influence or association of a specific culture. An example of this is how in the Chinese culture the color red may be associated with luck, or celebration, but in South Africa it is the color of death or mourning.
Resources:
AIO Lecture
http://www.color-wheel-pro.com/color-meaning.html
http://www.writedesignonline.com/resources/design/rules/color.html
http://webdesign.about.com/od/color/a/bl_colorculture.htm