AFive Shades Are Enough Already
If your map viewer cannot interpret your map at all or interprets it incorrectly, then
you have failed in your pursuit to communicate. In order to avoid this catastrophe,
the very first rule to learn in the Viewer’s Perception category of color rules is the
Five Shade Rule. It goes like this: the human eye can only distinguish between five
shades of the same color (hue). The correct application of this rule is as follows:
if you are emphasizing change in a variable over space with a commensurate change
in saturation of a color (i.e., choropleth mapping as explained in the next section),
you need to group your data into five or fewer categories. You must also take care
not to use any shade of that same color to denote any of the other features on the map
unless they are a completely different shape.