It kinda depends on what your goal is. Is it making a custom filament? or are you thinking of blending colors on the go? And are you looking for commercially available solutions?

If you’re looking for commercial solutions, then your guess is as good as mine. I know you can get filament recycling systems, where you crush failed prints and supports before then extruding usable filament. But I think that’s not what you’re looking for.

Going the tinker route instead, I think it was CNC kitchen that experimented with extruding their own filament by printing spiral of thick filament and then running it through a 1.75mm nozzle. They even got their logo embedded, and it was detectable in the finished prints layers.

But if you’re looking to blend on the go then maybe some manual g-code manipulation and custom hotends could work. But given that I asked about experiences with dual input hotend like the one below, and all the answers were along the lines of “don’t even bother”, I guess that you having to do a triple or quad input hotend won’t make it more likely to succeed.

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