actually we'll end up using natural colour filament thats colured with ink before it enters the hotend.

It's a much more elegant solution, there are several companies developing it, it has the potential for much greater precison, print speed and it's probably cheaper to run.
It's also a method capable of using pretty small coloured voxels and uses existing ink mixing technology.
And it can be bolted on to existing printers.

This is clever, and there are a few companies already selling machines with single nozzle multiple feed printheads - so it's not a new principle.

Good luck to him :-)