The 10+11, 9+10 are posterization levels. I take the same image, make a copy, and posterize each copy using different levels, then overlay a regular 2x2 checkerboard pattern over one image and pick up only the remaining pixels from that image and paste them over the 2nd image (same as the 15a, 15b procedure) Then IIRC, I posterize the lot to 8 levels.
I've been playing around with the PCE palette limitations a bit more too. Legendary Axe II always impressed me in the penultimate level with the large, concrete-ish columns that used about 32 colours with a base grey gradient. The artists did this quite cleverly (although everyone's done it in some way or another) by ramping the R,G,B bits in order of luminosity to make a few more shades in between each grey: Grey1 -> B->R->M->G->C->Y-> Grey2.
If you step this way through all the greys you get 50 shades instead of the normal 8:
Of course, the rainbowy pattern is an artifact that can either detract from or enhance the picture. Original low-colour pixel art probably is a better candidate for this rainbowy remapping than are renders or whatever.
Other ways involve using 15 or 16 colours with the in-betweens one hue or a couple.
Cyan
Red->Yellow
Some comparisons from pixel art, original by someone (not me.)
You all can let me know which one is more pleasing to your eye, & with reasons why.
First pic: Using just the 8 PCE greys; 2nd, using 16; 3rd, using 50.
The same with some converted render:
