The limitations of red-green colour scales in infographics
I used to love diverging red-green colour scales, but it's over now.
Map showing the differences in income of private households in different regions of Europe with the red-green colour scale
The reason for the popularity of red-green colour scales is probably the fact that they are so easy to interpret (at least in my culture, i.e. German culture). Green is associated with 'good' and red is associated with 'bad'. For instance, the map above shows the income of private households in different regions of Europe. I used a diverging colour scale to show the difference in income between households with average income (bright yellow), high income (green) and lower income (red). In fact, I used the exact color scale provided by the good old flare visualization toolkit.
But this scale has some limitations. Firstly, as alluded to above, the connotations associated with the colours red and green vary a lot from culture to culture. According to a helpful collection called 'Color Meanings By Culture', green has negative connotations in some Eastern cultures such as China or Indonesia, and red is associated with love, happiness and a long life in the same cultures.
The second limitation comes from the way in which someone suffering from dichromacy (also known as colour blind) would see these colours. Below is a simulation of how someone suffering from deuteranopia (green colour blindness) would see the red-green scale:
A similar problem is faced by individuals suffering from protanopia (red colour blindness), as you can see in the map below:
To me, these limitations alone are enough reason to give up using red-green colour scales for good. Instead, I'll be using blue-orange scales. After experimenting a few minutes with blue-orange scales, I found that hue values of 220° (blue) and 30° (orange) give a good result, as you can see in the map below:
Below is how someone suffering from deuteranopia (green colour blindness) and protanopia (red colour blindness) would see the map above in the blue-orange scale:
This scale is also friendly to someone suffering from a third type of colour blindness called tritanopia or blue colour blindness, as you can see in the map below:
For anyone who wants to produce on-the-fly colour blindness simulations, I would recommend a tool 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 colour blindness simulation. Some research is being done with that tool, namely 'Color Design for the Color Vision Impaired' by Jenny & Kelso, 2007. They're recommending using purple-green scales instead of red-green scales: