- :static-gray
 - Each pixel value indexes a grayscale value directly.
       - :grayscale
 - Each pixel is an index into a color map that maps pixel
        values into grayscale values. The color map can be changed by an        application.
       - :static-color
 - Each pixel value is an index into a predefined,        unmodifiable color map that maps pixel values into RGB values.
       - :pseudo-color
 - Each pixel is an index into a color map that maps
        pixel values into RGB values. The color map can be changed by an        application.
       - :true-color
 - Each pixel value directly contains red, green, and        blue components. Use the gdk:visual-red-pixel-details function,
        etc, to obtain information about how the components are assembled into        a pixel value.
        - :direct-color
 - Each pixel value contains red, green, and blue        components as for :true-color, but the components are mapped via
        a color table into the final output table instead of being converted        directly.