What is stopping us from mixing 3D filament colors in an Extruder? This came up in one of my groups today. That we could not color bend, or mix 3d printing filaments. I have researched but I am not finding anything talking about Plastic mixing in an extruder.

Why is it that we cannot take say a Diamond hotend, or a hotend with 5+ inputs, and mix any color we want? (assuming all the same type, ABSm, PLA). I think it would be interesting to at the least get a gradient effect on prints.

The best I have seen is natural plastic and a marker system. Or a powder / advanced / out of hobbyist price range process that sprays ink. The only Color Bending I know of is with Recycled plastic that uses multi color. Not quite what I am looking for.

Thanks!
I just started with google and phrase "3d printing color mixing" and on the first place (in fact first two were valueless adverts) I got this Instructables - DIY Full Color Mixing 3D Printer.

How it works?

It uses magenta / cyan / yellow filaments and mixes it while printing with Diamond hotend.

It definitely does what you are asking for and it's exactly the same idea you come up with ;)

Overview


  
  Getting the controller board ready for three extruders... I hacked
  a RAMBo board to drive three extruders, however, you can use any board
  you want... (most people use a RUMBA due to it having all the
  pins/components needed for 3 extruders native on the board) 
  Rewriting Repetier firmware to get color mixing working on your
  machine.
  How to install, configure, and use the diamond hotend - tips /
  tricks / lessons learned / etc...
  My original Bowden extruder design and various ways to mount the
  three extruders for your set-up
  My universal magnetic effector plate and accompanying hotend mounts
  for quickly swapping various hotends. (delta specific)
  How to design multi-color models and making STLs that can be
  exported and used as a individual STLs or combining them into an AMF
  file for slicing...
  Configuring color mixing in Repetier and Slic3r to print above
  mentioned multi-color models.
  Anything else I can think of later that I can't think of now.
  Comprehensive overview of Quantum Mechanical Entanglement as it
  pertains to multi-color printing (just kidding, I don't understand
  that... But I will cover multi-color printing throughly)