I used to love them, but it’s over now: diverging red-green color scales.
I bet the reason for the popularity of red-green color scales is that they are so easy to interpret (at least in my culture). Green means good, red means bad. For instance, the above map shows the income of private households for European regions. I used a diverging color scale to show the difference from the average income (bright yellow) with higher incomes in green and lower incomes in red. In fact, I picked the exact colors from good old flare visualization toolkit.
But there’s some trouble with this scale. Firstly, as alluded to above, the meaning of red and green vary a lot in different cultures. According to this helpful collection of Color Meanings By Culture, green is negatively associated in some Eastern cultures like China or Indonesia while red is associated to love, happiness and a long life.
The second source of trouble comes in if you want to include dichromats aka color blind people in your audience. Here’s a simulation of how someone with Deuteranope color blindness (aka green blindness) would see the red-green scale:
Almost the same problem for people with Protanope color blindness (aka red blindness):
For me, this is enough reason to simply forget about red-green color scales. Forever. Instead, I’ll switch to blue-orange scales. After experimenting around a few minutes, I found that hue values of 220° (blue) and 30° (orange) work out fine:
To make a long story short, here are the Deuteranope and Protanope simulations:
Also everything fine with the third kind of Dichromacy called Tritanope or blue blindness.
At the end I want to recommend a perfect tool for on-the-fly simulation of color blindess which is called Sim Daltonism. Many thanks to Michel Fortin for this. If you’ve heard of a similar tool for Windows and Linux, please let me know.
Color Oracle is another great tool for turning the whole screen into a color blindness simulation. Also, there’s an some research work coming with that tool, namely Color Design for the Color Vision Impaired by Jenny & Kelso, 2007. In short, they’re going for purple-green instead of red-green: