Model which I used to test multicolor printing within the same layer using single extruder printer without any multi-material unit.

THIS IS AN EXPERIMENT

PROCEED AT YOUR OWN RISK

The idea was simple:
We can create multicolor prints on single extruder single filament printers with tools like ColorPrint. You set the Z layer height in which you want the filament change, the tool alters the G-Code and the printer prompts you for the change once it reaches the layer height.

So.... How do I force the printer to stop mid layer for the filament change so I can print one model within the other? ie different model, different color/filament, same Z layer.

tl;dr setting, modeling, creation:

1. Select Slic3r profile for MMU printer in order to allow "Tool change G-Code" entry
2. Printer > Custom G-Code > Tool change G-Code
   Enter M600 into the text area.
   • M600 = pause for filament change
3. Printer > Custom G-Code > End G-code
   Remove the "If not has wipe tower" code block.
   • This would pull the filament into the cooling tubes in case of MMU, which is not a case for this project.
4. Prepare model for the first color
5. Prepare model for the second color
6. The models can NOT overlap!
   • in tools like Tinkercad, you can simply create a copy of the first model, change it to "hole" and then substract it from the second by merging them
7. Slic3r setting:
   • Printer > General > Extruders = 2
     • Leaving more extruders than you're going to use caused me an "incomplete G-Code" error
   • Printer > Extruder 1, 2, ...
     • I've copied setting from the single color profile as we practically have only one feeder and one extruder, so the Retraction options for MMU does not really apply
   • Print > Multiple extruders > Wipe tower > Disable
     • No need for wipe tower as the filament is changed manually
8. import both models into Slic3r and align them perfectly
9. set extruder associations
   • right click on the model > settings > Extruder = #
10. export G-Code

Printing:

• Please observe the process carefuly during the print and stop it any time you have an impression something went wrong.
• You need to stand by the printer, as you'll have to change the filaments manually.
  • Color selection is up to you. Whatever filament you provide to the printer will be used.
• There is something fishy at the end of the print. The model was completed, but the printer claimed it's not 100% done, thrown the filament out and performed few fishy moves, so I stopped the print manually. My suspicion is that "Pull the filament into the cooling tubes." G-Code in Printer > Custom G-Code > End G-Code, was the cause.
  UPDATE: It was the cause. After removing the section printer finished normally.
• Printer > Custom G-Code > Start G-code
  Something could be tweked in this section, as the printer creates a blob at start of the "Intro line". Just a minor thing, will work just fine if left as is.
• Theoretically you can set extruder for a different materials, so you could mix for example TPU and PLA, but you want to be carefull about the bed/hotend temperatures and the transitions.

Inspiration / How to
https://shop.prusa3d.com/forum/print-tips-archive--f86/printing-2-color-with-slic3r-t15054.html#p73052

Skull stencil author
http://www.chuckbauman.com/skulls-airbrushing-stencils.htm