There's a device called Palette, I think, that can do this on the fly. You hook it up between your filament(s) and your printer, and send it your print as well.

There's a device called Palette, I think, that can do this on the fly. You hook it up between your filament(s) and your printer, and send it your print as well. It basically looks at your print and determines how much filament is needed for each segment, and then splices together a few different inputs into one continuous thread. I don't know how well it works or how difficult it is to get setup, but the concept does exist.