Wouldn’t it be easier to code by doing the colors first, then assemble them?
Don’t know if I said this before, but you went with a big jump by going 3D. 3D is something I’d like to explore more in G’MIC myself.
Which makes me wonder, any goal in mind?
That’s what I did ( look at the castle part above).
I made a repeat loop to add each box with a different color each time.
But right now I’m trying to mod array_3d to add variation ( coords, rotation, size, color…what else?) but it’s not ideal since it assembles all objects after each x,y,z duplication. So I end up with lines of objects of the alternate color, etc. I had to modify it a lot and now it just look like my previous loop lol. But it works, just have to add the params. For example, you could just use it to spawn a whole patch of 3d grass or stones that way.
Build a procedural castle. Then a kingdom? At least the exterior. Then, if I use a seed, it can be explored in gmic_qt? Still, I have to learn how to use these triangles and quads to model basic stuff like arches etc. I could just use boxes though, will just look uglier.
It’s fun, join me? 
When I get to doing 3D fractal, I’ll definitely join you there. Xenodreams clone seems doable in G’MIC.
