Hemingray produced insulators in a rainbow of colors up through the 1920s. However, when you're talking about the value of an insulator, a slight difference in color can make a huge difference in value. For example, the colors "Hemingray blue" and "cobalt blue" can mean the difference of a $5 insulator and a $500 insulator!
Here is a simple color guide that shows some of the various colors that the Hemingray Glass Company produced. This is by no means a complete representation, but should at least serve as a starting point for basic colors. It's also important to note that some people will alter insulators to turn them into a different color. Please see the Fakes and Reproductions page for more information.