I've been tinkering with multi-material printing for few years now. My first printer capable of multiple materials was my FrankenCube beefed up up the Prometheus System. FrankenCube is originally a 3D Systems Cube 2, which has been heavily modified (rebuilt is perhaps a better word) and currently sports Duet 2 Wifi, 7" PanelDue, Prometheus v2 hotend and 4 Prostruders, enabling 4 colors/materials. The printer is in a stable state of will-be-finished-later so wiring etc. is messy.
The Prometheus System was from a Kickstarter campaign and originally supported only two materials. It seems the original Prometheus System is still available: https://www.distechautomation.com/pr...metheus-system.

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Prometheus a single nozzle-multi extruder setup and if I'm not mistaken, the first actual commercial offering. Prusa came along with their MMU around same time, but MMU is tied to their own printer, Prometheus instead being a kit that you can mount to your printer. Today at least MMU2S can be used on other machines, mainline Marlin supports it, but even Duet/ReprapFirmware has been hacked to work with it.

After a while of tinkering with the original Prometheus I went to 4x version. The changes needed were a 4x feeder part (https://www.thingiverse.com/thing:3807101) and a way to control more than 2 extruders when the board only supports 2. Luckily a solution had already been figured out by marlinkimbra.it: https://www.marlinkimbra.it/?p=280. The setup needs a 8-port relay (example https://www.amazon.com/SainSmart-101.../dp/B0057OC5WK), some wire, 5V, ground and 3 pins from controller board to control the relay.
I've used Marlin and ReprapFirmware to control the relay; on Marlin, some code changes were needed, on RRF it's just few extra lines to tool change gcode files.
See Duet forum for more detailed post about the FrankenCube: https://forum.duet3d.com/topic/7592/...=1595193566063
Some pictures along the way of tuning the Prometheus: https://drive.google.com/open?id=19-...GbK8JzFg3tGf9o
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