A form may be 2 dimensional or 3 dimensional. A 2 dimensional form is easy to recognize because it is unshaded and flat in appearance. This flat shape can be any color or no color.  The shape itself has the appearance of a silhouette  when seen against another color. for example, black against white. 
        Sometimes very simple flat shapes can be transformed into stunning 3d masses. Sculpture is made by artists who have a special fondness for the appearance of 3 dimensional mass. Sculptors like the way surfaces change and flow into other surfaces. They like the way light reflects from the surfaces illuminating them with light and shadow.  Here are examples of 2d transformed to 3d.