Better colour distribution in radial tree diagram

[simongray]

(continuing work started in #31)

The colours of the radial tree diagram are currently distributed in such a way that similar colours often end up in close proximity to each other (they're just in the alphabetical order now).

Perhaps it is possible to sort them via colour distance when creating the diagram? E.g. pick the next sort option according to the max colour distance. Obviously, this is somewhat computationally expensive.