How the map is made
The DARC radius search lists the nearest local clubs for a place, each with its own postcode. The postcodes become coordinates, and the coordinates become the partition: each cell is the set of all points closer to that seat than to any other. Mathematically it's a Voronoi diagram, and it's exactly the same calculation the search by location does, just once in advance instead of on every request.
Every district has its own colour, and where two areas meet, two clearly different colours are always side by side. This is assigned in two steps: first each district gets a colour family so that no neighbour has the same one, then its own brightness within the family. Deriving it from the letter would be simpler but look worse, because districts next to each other in the alphabet are usually neighbours on the map too and would then look almost the same.
That a district's cells hang together almost everywhere isn't built into the map, it comes from the seats alone. Where a single area is out of line, a local club really does sit in a different corner from its district neighbours.
What's missing are the VFDB local clubs, i.e. district Z. The DARC search doesn't list them, and nothing gets guessed here.