Gradient biased to black (it seems); why? Working on the solution for How to use transmissive voxels with color gradients? to be transmissive instead of reflective, I have a problem that I do not understand (Blender 2.79):

The gradient that is meant to be linear does not blend in a linear way in my eyes. Instead the gradient seems biased to wards black or darker colors. Even when I try to apply gamma correction, the gradient does not improve.

As I'm a beginner with blender, it's possible that I mad some mistake due to misunderstanding, so please tell me what I did wrong, and preferably how to fix it.

First here are the colors that should be blended (basically an RGB cube):


And here is how my attempts look:



Remark: The first image on the second row does not match the view angle of the cube in the first row, but the second one does.
Finally, this is my node setup (hoping that's enough for the expert to understand):



The result I'd like to see should be as smooth as this if possible:



Unfortunately the emissive version does not have smooth gradients:
In vertex color paint one can see the useful interpolation.  This may or may not be useful.  If you just want the gradient you could vertex color paint simple planes and use them as textures.  Blender Render.  Arrow indicates required setting.

For your example try to avoid negative coordinates relative to origin.



Nodes with exponent of 1.   Origin of cube in center of cube. Image above.  Negative Values possible for coordinates.  Not desirable for your goals.





Origin of cube is in the corner. Image Above. Zero or Positive Values for coordinates.  This origin works with your goals.



Make sure your placement of the cube has 

one corner at (0,0,0) and origin is there as well.

one corner at (1,1,1)



You may want to research Barycentric Interpolation