I've seen a model on Printables.com (Ninja Pot by @Wilko) which didn't show the standard orange color for the whole model. This works because the 3MF file format supports colors. Afaik some slicers even support that information for multi color prints, but I'm not sure about that.
There are a few solutions how to do that (a plugin for blender, another one for OpenSCAD, afaik Netfabb), but I decided to write my own. Hopefully there isn't a totally obvious solution available that I overlooked ^^