newb question dual extrusion

If I design an objet in Blender, Do I state in Blender the colors to use or is it programmed somewhere else?

Or said another way, how does the printer know what color/extruder to use when printing?

We typically slice a unique .stl for each color - one for red parts, one for blue parts, etc., then designate which head prints which gcode (based on what color or material is loaded).

OK so draw 2 parts in Blender, export 2 stl's. How do you put them together in slicer?

Here's an example of doing it on Cura. If your printer is open sourced based, it will probably work.

https://www.youtube.com/watch?v=FKgHEO1g-S0

Thanks! That put it into perspective for me.