How do I use a 2D polygon collider to generate a correct collider for my randomly generated mesh?

Currently I am making a randomly generated 2d tile based game in which a map generator object creates a large number of chunks and then each chunk generates a square of tiles. I have the random generation(or at least part of it) but the idea of collision evades me. The object is a mesh with each tile consisting of 2 triangles but it seems like a polygon collider where I feed it the vertices is the best option since not every tile is its own gameobject and I can't just put box colliders on all of them. My question is how would I use a polygon collider to make the darker regions have collisions while the lighter regions not have collision?

Since polygon colliders have one array of vertices I was wondering how the component determines what the inside of the collider is and how to use a segmented mesh with a polygon collider because at some point my random generation would lead to a chunk being split in two.