Turn the stone in the light. Some tourmalines appear to be different colors depending on which way the light is hitting the stone. Rotate your stone in the light and view it in a variety of lights to determine what shades are in the stone. You may have a stone that is more than one color, or you may have a stone that is always within the same color but has different shades. Generally stones that have the same color in different hues are more valuable.
Examine the color gradient in the stone. Some tourmalines have a very clear divide between colors. For example, one part of the stone might be green and the other part red. This type of tourmaline is called a bi-color tourmaline and is often used for artistic jewelry or as a conversation piece if the stone is large enough.
Examine a cross section of the stone. Really variable tourmalines are sometimes cut in cross sections so that you can see every color inside. You will be able to see two or three different layers of color. This makes it easy to tell if a stone is bi-color, tri-color or a unique type of tourmaline called an aschentrekker, which is pink on the outside and black on the inside.
Look for a deep green color. Tourmalines that always appear in varying shades of green are verdelites. However, if the stone shows flashes of blue when it is rotated it is a rare Paraiba tourmaline, whereas if it just keeps shades of green then is a more common but still popular "chrome tourmaline."
Check for stones that are bright yellow. These stones are called "canary tourmalines." They can have brownish tones or be a pure, sunshiny yellow.