Tourmalines occur naturally in every color of the rainbow, sometimes even two colors at once. In fact, tourmaline crystals are more likely to have two (or more) colors than just one--or at least two or more tones of one color. Even minor changes in a tourmaline's chemical composition can serendipitously result in entirely different colors within the single crystal. When tourmaline grows so that the center is red with white or colorless material around it, and green around both of those, it's known--not surprisingly--as watermelon tourmaline. It's usually cut in thin slices or cross-sections to best show off its interesting color growth. Tourmalines from Malawi, known as canary tourmaline, are a vivid yellow color.
In addition to usually being bi- or tricolored, showing two or more colors from one view, tourmaline is also strongly pleochroic, meaning a stone that appears to be only one color face-up can appear to be two or more colors, or tones of the same color, depending upon which direction it is viewed. Look through the gem's table with a dichroscope and you'll see both of tourmaline's pleochroic colors.
Cat's-eye (chatoyant) tourmaline can occur in many colors but particularly in green and pink varieties. Some tourmalines also display color change, from yellowish-green in daylight to orange-red in incandescent light.
Tourmaline is sometimes referred to as the "electric stone" because it becomes statically charged and can attract dust, lint, or small bits of paper when it's heated or rubbed. This unique characteristic is referred to as pyroelectricity (or piezoelectricity). For this reason, tourmalines and tourmaline jewelry might get "dusty" and require slightly more frequent cleaning than other jewelry and gemstones to keep them looking their best.