It uses colour to show all the states in the region in 1789. This date is significant because it is the eve of the French revolution. Within a generation, Napoleon would have made huge changes to the states of Germany, greatly reducing their number. All the church lands (shown on the plate in purple), all but four of the Imperial Free Cities (shown in orange) and many of the smaller lay states would be abolished and their territories assigned to lay rulers.
However this plate not only shows the many states existing in 1789. It also shows, by red lines, the contemporary borders. These reflect further changes that occurred since 1815. For instance, in 1815 two Imperial Free Cities, Frankfurt and Bremen, remained in the region covered by the plate. But by 1905, Frankfurt had lost its independence to Prussia in the war of 1866.
To make it easier to see what these red borders indicate, I have added partly-transparent overlays showing the 31 territories which they demarcate. Six of these are Prussian provinces, 22 are other states within Germany, and three are states outside Germany. You can use the buttons arrayed below this text to enable or disable each overlay. The overlays are (or should be, depending on your browser) transparent so you can see through them to the original plate. You can also hide the original plate so that you can see these contemporary states more clearly.
The "four-colour map theorem" states that four colours are sufficient to colour any plane map. This however assumes that all the countries are "connected", that is, they do not have exclaves. The contemporary map presented here requires six colours. The six countries that force the use of six colours are indicated by + signs.