You have a few possible graphical patterns to express that several entities belong together:
(source: Colin Ware, Visual thinking for design)
You have already used color with one meaning. Overloading color with a second meaning would make the cognitive effort for extracting information from your graphic much higher. So I think that the "common color region" solution should not be used in your case.
You are already using "enclosing contour". I like Alexey's suggestion of making this more obvious, by strengthening the contour. I would also add a "proximity grouping" to make it even more clear which entities belong together, which simply means that I'd put more whitespace between provinces.
Another matter you didn't mention in your question: the order of the information. When people look for information, they need some order, or they get lost. Sometimes information has a natural order, and then you should use it. I would strongly recommend structuring your graphic layout in the same way the provinces are geographically structured on a map. This doesn't mean that you have to make your elements the same shape as the provinces, or that you have to exactly follow the distances and sizes of provinces in your layout. But the relative position between province data can be made the same as on a map, helping your users orient themselves without losing the curved aesthetics of the graphic.
Update I just saw the comment about a "spiral" you posted while I was writing the answer. It is an interesting idea, but I personally didn't notice it, and from the other answers, it seems that others didn't notice it either. Maybe you could use more whitespace again, to make the spiral. But it is very unusual for a spiral to have its largest elements in the middle; practically all natural spirals have the smallest elements in the middle (take a look at a nautilus shell, or at the spiral of square roots). So I think it will be hard to make this well visible, unless you make the elements much smaller, so the geometric information becomes much more salient than the colors and the shape of each individual feature. But in this case you would be deemphasizing the information on the detail level, which is probably not what you wanted to achieve.