Nice. Interested to see how the color mixing works. I tried it on my clone cyclops hotend with varying results, at least some PLAs weren't mixing, but resulted in a more 'toothpaste' type extrusion. Apparently PETG should mix better, but I didn't try ir, the hotend had too much issues and I didn't have anough patience with it...

About slicers: for MMU2S I think PrusaSlicer is probably the easiest to start with since it the multi-material support is written MMU2S in mind. S3D is harder to get working, at least on my setup where I need to use post-processing sw to add the actual tool change gcode after slicing.
For color mizing I don't know any slicer that supports it. For my tests I wrote a short python port-processor to add the mixing ratio commands per layer.
IMG_20200724_103718.jpgIMG_20200724_103727.jpg

Multi-part desings: if you can read OpenSCAD, I designed some multi-part prints that I use when testing my post-processor: https://github.com/spegelius/3DModel...estmodels.scad
Basically the design process involves designin multiple parts and cutting part y from part x when they overlap, making sure the model origins match so that when imported to the slicer the models are in corrent positions. In slicer then just assing models to proper extruders (in S3D this is a kinda PITA to do properly)