Contains a list of all the items which have an assigned color, and the list of all the different color categories which exist. VMD uses a colormap made of 16 base colors (plus black), and a colorscale made of 32 colors in user-definable gradient (e.g., red-green-blue, or red-white-blue). Color categories are used to contain the names of items which are related and which may have their colors changed; an example of a color category is `Axes', which contains the colors for the `X', `Y', and `Z' axes. The categories contain just a list of names and the associated colors assigned to these names (an integer, from 0 ... 16). Other objects in VMD which are derived from ColorUser can add new color categories (or can query if one of a specific name already exists), and can add new names to an existing color category. The colors assigned to the named items in each category can be changed via the Color menu, or through the color text command. Once a category is created by some ColorUser object (or, if it already exists, has been obtained by the object from ColorList), the ColorUser can use the category to retrieve the proper color to use for whatever purpose it needs it (i.e., creating the object's display list if it is a Displayable).