Minuteman on youtube has a four extruder single nozzle setup for speed printing. He occasionally uses multiple colors, and they basically don't mix in the nozzle. The cross section of an extrusion looks like a pie chart

You would need special nozzle geometry that encourages mixing, and likely much higher temperatures. And any such mixing geometry will trap some filament. Switching from dark to light colors might require purging with meters of light filament to get all traces of black filament out