I think it's more to do with each 16x16 area of tiles needing the same palette.  Something like that anyways.   Still it looks nice. :)  NES has some of the most complex and headache-inducing graphical restriction rules ever so it's even more impressive when people make quality stuff with it.  The best NES graphics back in the day were usually done by those folks who had practiced the hell out of it and made it work for them.

Hmm.  Each 8x8 tile uses one background pallet.  I think I went wrong with having more than four background pallettes.  I was mostly practicing the color combinations so I missed this...  I'm starting one more mock up and I'll pay closer attention to this detail.

You don't seem to following the attribute table grid (1 palette per 2x2 tiles).