Introducing Phosphor

I am proud to release Phosphor, a new tool to make color for data visualization even easier.

Why We Need This

We see and make use of color every day, to the extent we often take it for granted. But color is actually a hard problem, and even computers are bad at color.

Our eyes are incredibly sophisticated and the way we perceive color is much different from the way screens display color. And with all things human, there is incredible variety in the way people perceive, understand, encode, and use colors. From differences in the perception of color due to human physiology and diversity in the tools we use, a truly useful color ramp is one that accounts for both how we see color and how we intend to use color.

Phosphor attempts to solve a few problems:

1. Color ramps should be perceptually linear. This means that the perceived increases or decreases seen as we move across a color ramp are congruent with the data being represented by that color ramp. The components that make up a color (a common model is based on hue, saturation, and value) are not equally appropriate for representing quantitative data. For instance, red is not inherently ‘more’ or ’less’ than purple or green. But there is a less -> more relationship in value: How dark or light something is can be tied more closely to quantities.

   A color ramp that is all over the place, going from light to dark back to light and then a different shade of dark, cannot faithfully represent any data. The changes we see are in the color ramp itself, which then either mask or exaggerate changes in the underlying data.

   But when we have linear changes from light to dark (or dark to light), then we can see changes in the data how they really are. Small changes are subtle shifts in lightness. Large differences in the data are larger differences in lightness. Similar data looks similar, and outliers truly stand out.

   This is a well documented subject in data visualization.

2. Color ramps should be accessible. A sizeable portion of people have some form of colorblindness, with protanopia and duteranopia (red-green colorblindness) being the most common. Global estimates suggest nearly 300 million people are affected by colorblindness. While this varies with geography and gender (red-green colorblindness is a genetic trait of the x-chromosome), a widely accepted statistic is that about 1 in 12 men experience colorblindness. If you produce any kind of map or visualization for an audience, you can be sure that some of them do not see it the way you do.

   Well-designed color ramps solve this in part by being perceptually linear. Since we don’t use hue to represent quantities, the relationship between light and dark remains the key attribute of the data visualization. We can design even better palettes by avoiding greens and red of similar lightness, and opting for alternative hue combinations instead.

3. Maps and data visualizations come from many tools. We all use and prefer different software. Some projects, or personal tastes, make use of multiple tools in a single workflow. A useful color ramp should be able to be replicated in the tools you use most.

   Choose or make one palette, then use it everywhere.

Enter Phosphor

Phosphor was designed from the ground up to address each of these challenges.

1. It automatically corrects the linearity of your chosen colors. Whether you pick 2 or 20 colors, Phosphor interpolates between them to produce a linear change in value. The default interpolation is based on the Oklab color space, but other options are available, too.

2. It simulates how your palette would appear to someone with common color vision deficiencies, which include protanopia, deuteranopia, tritanopia, and achromatopsia (complete loss of color perception).

3. It exports the color ramp you’ve chosen or designed in a number of popular formats:

   • Raw hex values
   • CSS
   • Python/Matplotlib
   • ArcGIS Pro styles
   • QGIS styles

And importantly, all parameters of a color ramp are built into the URL so you can easily bookmark color ramps for later use, or share color ramps with others.

Phosphor comes with a number of pre-defined gradients, from those I’ve produced during my time at NASA to popular ColorBrewer sequences. You can start from scratch, or modify these color ramps as you like.

Best of all: This is just getting started. New features, color ramps, and formats will be added frequently.

Give Phosphor a try. And please let me know if you run into any bugs or have any feature requests.

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