As a (plain M'brot) iteration progresses, the Julia Set / z values vary with a period that's characteristic of c.
(The period can be determined by noting the iterations at which z is smallest (or largest, or has the smallest real component, etc.)
The period divides (inner-) M-space into regions of equal period.
Those periods match that of the buds on the ~border~ of M.
And, so, coloring them (solid and) distinctly makes a nice and illuminating picture.
One could also display the period at a c-point as a Tool Tip (say).
Does XaoS do anything like that?