Free Map Maker: Color a US Map

What Is a Color-Coded (Choropleth) Map?

A choropleth map (derived from the Greek chōros “area/region” and plēthos “multitude”) is a thematic map in which geographic administrative units—such as states, counties, provinces, or nations—are shaded, patterned, or colored in proportion to a statistical variable.

First developed in 1826 by French mathematician Charles Dupin to visualize literacy rates across France's departments, choropleth maps remain the primary visual vehicle for communicating election results, epidemiological disease transmission, household income, and environmental metrics.

Transforming a blank boundary map into an authoritative thematic presentation requires balancing mathematical classification, human optical perception, and topological rigor.

Why Four Colors Are Enough to Color Any Map

When coloring a blank map to distinguish neighboring jurisdictions (such as coloring the 50 US states or European nations), what is the minimum number of colors required so that no two adjacent territories share the same hue?

This is the famous Four Color Theorem:

• The Mathematical Law: No more than four colors are required to color the regions of any planar map such that no two adjacent regions (regions sharing a common boundary segment of non-zero length) have the same color. Regions meeting only at a single point (such as the Four Corners in the American Southwest) are permitted to share the same color.
• Historic Computer Proof (1976): First conjectured in 1852 by Francis Guthrie, the theorem defied mathematical proof for 124 years until Kenneth Appel and Wolfgang Haken of the University of Illinois solved it in 1976. It was the first major mathematical theorem in history proved using a computer algorithm, analyzing 1,936 reducible graph configurations across 1,200 CPU hours.

How to Group Your Data Into Color Classes

When mapping numeric statistics (like median income or disease incidence), the cartographer must group continuous data into discrete color bins (typically 4 to 7 classes). The choice of classification algorithm completely alters the visual narrative:

How Jenks Natural Breaks Are Calculated

To quantify how well a classification scheme represents the underlying statistical distribution, cartographers calculate the Goodness of Variance Fit (GVF):

Color Map Classification Methods Compared (Table)

Reference this decision matrix comparing the five standard statistical classification methods in thematic cartography:

Choosing Accessible Colors and Avoiding Misleading Maps

Professional cartographers adhere to strict graphic design and ethical guidelines when styling maps:

• Cynthia Brewer's ColorBrewer Standards: Cartographer Cynthia Brewer developed the industry-standard ColorBrewer palette system. Palettes are categorized as Sequential (light-to-dark hues for continuous numerical data), Diverging (two contrasting hues like purple and orange meeting at a neutral white midpoint for deviations from a median), and Qualitative (distinct hues for nominal categories).
• Color Vision Deficiency (CVD): Approximately 8% of men and 0.5% of women have red-green color blindness (deuteranopia or protanopia). Avoid red-to-green palettes; instead, utilize blue-to-orange, viridis, or cividis scales that maintain unambiguous luminance separation.
• The Modifiable Areal Unit Problem (MAUP): In choropleth maps, large rural counties (like San Bernardino County, California, covering 52,000 km²) visually overwhelm the map canvas, creating the false optical impression that rural characteristics represent the vast majority of people, while tiny, densely populated urban counties (like New York County, 59 km²) disappear entirely. When mapping absolute counts, normalize by population or land area.