Thanks so much for your advice - I tried M0 and that did nothing, so I found out that my machine (Prusa mk2s) uses Marlin which has the M600 code for changing filament. Unfortunately that also seems to do nothing, and I think here's why (and what I'm asking for now): So I am testing printing 4 process on one layer that I want to all be different colors. They are intertwined, so using the G1 options won't quite do it. In S3D I'm telling all of the processes to slice at once (and it's printing them separately and correctly so long as it runs all the way through with no pause/change), but when I put M600 in the starting script of the 2nd, 3rd, and 4th processes it does nothing (though connecting to the printer and sending M600 directly to the printer gives me the result I'm looking for). I'm just guessing here, but it appears that S3D is NOT looking at the individual process scripts once I have selected more than one and sliced it - but possibly creating a single starting and ending script based on the first and last models? Are there any additional thoughts about this, or suggestions on getting my intended result?
Edit: I have a work-around that (unfortunately) doens't have much to do with S3D. I saved the GCode and then manually edited it. S3D calls each process a "layer" and I was able to add a line for "M600" after each new "layer".