Marimekko: assign colour and label to regionless entities

[lucasrodes]

Core problem

reference: https://github.com/owid/owid-issues/issues/643#issuecomment-2082701311

At the moment we can't assign a color and title to the group of entities that do not belong to any group.

Proposed solution

We can show entities that do not belong to any group by unchecking "Exclude entities that do not belong in any color group". However, these are shown with a color blue which is very similar to the one used by Europe. I think that we should be able to choose a different colour from the Admin.

Also, it'd be nice if we could assign these entities to existing groups (e.g. like the case of former countries / or ad-hoc regions like 'Grand Duchy of Baden', etc.) or create a new one (e.g. 'Europe' as part of 'World' or 'High-income countries' as part of 'World Bank income groups').

Note that showing regions and countries in marimekkos only makes sense if no indicator is used for the x-axis (as in this one)

[danyx23]

Chatted about this in the triage meeting - we think that an easy quick first fix would be to use a default color that is further away from the color for the continents.