Looking nice, BUT, this would not work on a real NES as it only has one layer.
Also if you would wanna go behind something that is not a full 8x8 tile (as in partly seethrough), I thikn one of the colours has to be fixed as transparent. Doing this with the same 4 colours but different colours as transparent would be a new palette each time. And I think you know you only have 4 of those.