Making a map of the closest capital using QGIS

I saw this map on Facebook. It asks what capitals are closest to each Italian town, creating what looks like a continuous map to display the result. I like this kind of thing, so I set out to replicate it using QGIS, and making the process as replicable as possible. The TL;DR is as follows I first got the data and boundaries I found a formula to calculate the closest capital by distance I found a way to apply a parametric colouring to the map.…