I have done what you need many times for other designers models and for my own.
You can read about it here.
The trick is to benefit from being able to unload filaments from the AMS or AMS lite which are not the one currently being used to print.
It does mean you need models that have a maximum of 4 colours in any one layer, but, can have many more colours within the z-axis.
The bear example I provide is a good one as it is simple to understand.
You look for colours that can be removed as they are not used when you need to utilise a different colour.
In the bear example, the red filament wasn’t used in the top half of the model, so if you make the eyes red, you are simply telling the slicer that whichever slot holds red will be used when it gets to the eyes.
All you need to do is swap the red out its slot and place the white in the same slot.
You make this easy as you add a PAUSE at a layer height between where the red is last used and before the faked red eyes will actually be printed.
As red isn’t being used, it can easily be removed from the printer and the white swapped in, RESUME the print and when it gets to the eyes, the white is used.