So the first request was different materials by MoGrapgh Selections... and the principles behind moving to Spherical fields is pretty much identical. Bear with me @HappyPolygon as I'm going do a little extra explaining for any new users searching for this in the future.
It's worth noting that OP wasn't able to control MoGraph the way he wanted because he felt restricted to solid colors. Solid colors are actually your friend when it comes to MoGraph... more importantly... Black to White... which is the same as 0-1 (0%-100%). When you put 10 images in a MultiShader, the first one will be used on any clone that is in the range of 0->.1, the second .1->.2, etc (assuming it's in Color Mode).
So I have 4 materials I want to switch between in my cloner. So I start thinking in 4 divisions of 100%... 0-25%, 25-50%, etc... That way the cloner will use the first clone 0-25%, the second clone 25-50%, etc.
These divisions are ultimately "value divisions" and the reason why I call them that is because mograph takes the final "value" of the clone that can be determined by many factors including the Initial Cloner Color, Effector Weight, Effector Strength, and Field Opacity to name a few. This particular example starts everything at 0 and adds value through the Field Opacity, but there are many ways to get here..
You will notice that one spherical field will always dominate another when they intersect just as one color will dominate another depending on the layer order and blending mode. But now that we understand what we're doing with color it shouldn't be hard to see that we can simply rearrange the layers to change dominance or if we wanted even more specific materials where our spheres intersected we would simply need to divide our MoGraph values into more parts.
And if this makes sense in 2D... it holds true when you move to a volume.
Cheers.
MoGraph_Material_Fields.c4d