Note: This node is based on traditional color theory. As such, it bases its statements on the traditional color wheel. If you feel you prefer green over yellow as your primary color of choice, you might want to take this into account while reading.
A color scheme is an arrangement of specially chosen hues of color designed to evoke a certain reaction. Sometimes a color scheme will revolve around a specific hue (such as blue) and it's local shades (light blue, navy blue, ocean blue, sky blue, etc.) and focus on the shades within a given spectrum (only darks, only lights, middle colors without extremes). However artists tend to prefer using a variety of color to achieve their hoped for effect. It is common to base a color scheme on a "warm" theme (red, orange, yellow, sometimes purple) or a "cold" scheme (blue, green, sometimes purple), and it's not irregular to use both in different parts of an image to add especially strong contrast.
While the "warm" vs. "cold" scheme is based entirely on the human eye's poor recognition of the color blue one of the other important elements of scheme design is the concept of complementary colors. Complementary colors are "opposites" that sit across from one another on the color wheel and often have high shock value. Examples include purple and yellow, red and green, and blue and orange. Complementary color schemes create an unnatural, sometimes surreal effect. Thus they are most often used when an artist wishes to confer an alien theme to an image or to draw the viewers eye to a specific point.
Complementary and "warm" vs. "cold" schemes are both examples of bipolar color schemes - schemes with two very specific themes. They have been provided as the most basic examples of how schemes work. In reality, a color scheme is just an arbitrary term for all the thousands of color choices an artist can make. By no means should you ever limit your work to just two color choices. Be creative, intentional, and most of all, unorthodox in your choice of color.