Grey unicorn is the final stage I think
After grey unicorn, there’s the brown unicorn. Basically, the rainbow donkey is the last donkey tier, and you start again with the unicorns all the way until you get the rainbow unicorn. Also the only one that can fly.
I used to keep my donkeys in individual cells, only one block wide, using doors. I got a nice mix of babies, mostly the same colour as the mother, some the earlier colour, some the next colour. Most of them were tamed and well-fed (manual feeding). I don’t remember if any had babies before I tamed them.
(Dodos need more room in each cell, at least two blocks, even with gates rather than doors, even when tamed and well fed.)
Good luck with the unicorns though. They are, in a way, harder to breed. Plus really, really expensive.
Now I’m planning a set of experiments. Width of cage. Number of donkeys in it. Type of feeding (manual as soon as hunger bar shows, manual when bar is half red, feeder chest). Record time and colour of babies.
(Adds yet another page to my “do if have infinite time” list.)