A park map that quietly explains why your charts are hard to read

Look at almost any well-made hiking map and you'll notice something you never had to be taught: blue means water, green means park land, red means the road you should take to get there. Color and visualization expert Maureen Stone uses exactly this kind of National Park Service map, of the Point Reyes area in California, to explain what she calls functional color, meaning color chosen for a purpose rather than for decoration.

On that map, the red highways contrast in both hue and lightness against the pale background, which is exactly why your eye finds the road network first, the way the map's own function demands. Labels sitting inside the pale blue water are printed in a slightly darker, more saturated blue, which creates a quiet second layer you can read without it competing with the roads. None of this happened by accident. Every color choice served the map's job: helping a driver find a route.

Stone offers a useful test for any colored chart or slide you make: convert it to grayscale and see if it still works. Strip the Point Reyes map of its hues and the roads still dominate, because they contrast strongly in lightness, not just in color, against the land around them. Designers call this discipline getting it right in black and white, and it matters because between eight and ten percent of men and roughly one percent of women have some degree of color vision deficiency, and they read lightness contrast just fine even when they can't distinguish red from green.

Edward Tufte opens his own chapter on color, in his book Envisioning Information, with a warning Stone echoes: above all, do no harm. Color used well can clarify and beautify. Color used badly, applied heavily without a clear reason, can make an otherwise fine chart worse than if you'd left it in black and white entirely. The point was never how many colors you use. It's whether each one is telling your audience something they needed to know.

Before you add a single color to your next chart or slide, decide on paper what job each color is going to do, the way a park map assigns blue to water and red to roads and never mixes them up. Once you've built it, switch the whole thing to grayscale and check that the important shapes and contrasts still hold without the color there to prop them up; if they collapse, fix the underlying design before you touch the palette again. Wherever text sits on a background that shifts in lightness, double check it stays readable in every position, not just the one you happened to preview. And be ruthless about using the same color only for things that are genuinely related, because your audience will assume a connection exists the moment you repeat a hue, whether you intended it or not.