How to make a two-way colour ramp with QGIS, R or similar?

Many recent maps I have seen have these neat two-way color gradients that can be used to color correlations between continuous estimates.

Here is an example taken from a recent publication (Source: http://ift.tt/1Eu93Os ).

I believe those gradients have been generated with ArcGIS-10-x and I was wondering how I could reproduce such color gradients with freely available GIS tools (or at least those available on Linux)

Happy to hear suggestions or code snippets.