Start by opening a small map, then turn off every color except gray. Then figure out which layer carries the main message. This could be a layer that shows population groups, land use, elevation bands, or something else. Bring back color just for this layer. Leave roads, borders, water, and other base map features uncolored. This helps you see what the color palette should do before individual colors start fighting with each other.
A good color palette should define roles instead of picking colors based on preferences. Figure out which features should come forward, which should give geographic context, and which should be nearly invisible. The thematic layer typically requires the strongest visual differentiation. Base map features can have very light fills, light lines, and low saturation. Labels need to be legible across both light and dark areas. Map title, source note, and scale bar should help the map but not draw attention.
Try to limit the initial draft to a few colors. A lot of bright colors makes it difficult to distinguish between categories, and can lead to accidental emphasis. For sequential data (for example, low-to-high), use a progression of colors that change gradually in lightness or saturation. For nominal data (categories without an order), choose colors that are different from each other but have similar intensity. A bright red category next to several dull colors will seem much more important than the data actually suggests.
People often check colors on a big computer screen, zoomed in close. That way, differences between colors might be obvious, but they won’t be when the map is shrunk down for a presentation, website, or newspaper. Create a draft at the final size and view it at normal size. Make sure that neighboring areas are distinguishable, that thin lines don’t disappear, and that labels are still legible on top of every possible fill color.
Another option for checking your palette is to render the map in grayscale. If two categories have similar lightness, they will look identical in grayscale even though they appear different in color. This is particularly relevant for people with impaired color vision, or for a map that has been printed in black-and-white. You can differentiate the layers using lightness, hatch patterns, or symbol styles. Use the exact same fills and lines in the legend as on the map itself so that each category is easy to identify.
Once you have a working color palette for the main features, introduce additional colors sparingly. You can use a cool blue-gray for water, neutral colors for roads, and a lighter color for administrative boundaries. Don’t pick these colors individually. A beautiful blue for water could be too vivid next to the data palette you have chosen, while a bold boundary line could overpower its neighboring regions. Look at the whole map, and mute any colors that draw attention for the wrong reasons.
The best color palettes aren’t necessarily flashy. They’re successful when the map’s most important information is immediately understandable, neighboring categories can be distinguished from one another, labels are legible everywhere, and the geographic context is clear. Once you can articulate why each color is there without making reference to subjective preferences, you have reached the point where the color palette is an integral part of the map.
