We can define a continuous number x of iterations needed to reach the critical radius R. Note that if the n-th iteration of f(z) equals R then x=n, and if the (n-1)th iteration equals R then x=n-1. For values in-between we use linear interpolation on R to get x with these two reference points. You can discover this in “self-similar fractals … – the code“. The continuous number x then defines the hue of the pixel with colormode(HSB). Look at the calculation for the variable “itecon” and the function “color rainbow()”.
This is an example for the results you can get. I used the function with the inversion on z: