cover: Use coloured diamond, but actually, the wall and element layers are separate, so any interaction with them is possible, but it was not meant like that.
Proof:
pmap[y][x] Particle-map. Returns particle at [y][x]
bmap[y][x] Block-map. Returns wall at [y][x].
https://powdertoy.co.uk/Wiki.html?id=variables

That's not what I discussed. The point is that walls would have different deco if used. Also, walls are inert so for example, no element can pass through or even be put into a wall.