I like the style.

I am in two minds whether to add a second plot with a newly registered color sequence or to leave this as a purely reference example of what is available by default.

You can add a sentence that users can register their own sequences as well and point to register. IMHO an example is not too helpful here. The concept "register -> you can query it by name" is straigt forward. The plot itself will not be helpful as you cannot visualize "there's a new element in the registry". That can only be inferred from the associated example code. But as said, the concept is so simple that any example is dilluting the message because of the additional visualization code.

Minor style suggestions.

Take or leave the suggestions.