Overlay lattice on background

I’m trying to achieve the following result.

overlay

Where a material fills the space between lattices, but the space outside is filled by a different material.
I can’t seem to find the correct method.

I tried to create a case where a lattice and set lattice.outer to an appropriate universe

background

But the result was that the entire circumscribed hexagon became blue, not just the space between lattice positions, as intended.