true color printing with CMYK+White (not 3-in-1 diamond head RGB)
You may be a bit misled here. First of all, you do not want "RGB" , as those are additive colors such as used when combining light sources.  You do want "RYB" (red-yellow-blue) or the more accurate CMY(plus K just to get a 'truer' black)  for subtractive colors.

Next, there's really no reason to attempt pixel-mixing. What should happen, ideally, is that pigments get fully mixed upon extrusion so that the desired color is actually in place. Pretty much any pixel-based setup will not "blend" into the desired visual perception.  And as you propose, you really need a White and a Black to adjust the saturation (take a look at the Wikipedia pages on Hue and Saturation color maps).
So I'm not convinced that separate extrusion heads will ever get you a decent color continuum.  I don't know if anyone has, or is planning, a multi-input, single-output head but I'd sure like one.