Does this replace vanilla pipes?

No. It adds a separate tier 2 pipe to ground.

Why are the visuals darker now?

The first release used vanilla-looking sprites, which made it too hard to distinguish tier 2 pipes from the original pipe to ground. Version 0.1.2 pushes the tier 2 pipe further toward black anthracite for stronger in-game contrast.

Does this change the reach?

No balance change in 0.1.2. It still has twice the vanilla underground reach.

Is this safe for existing saves?

Yes. Existing tier 2 pipe-to-ground entities should keep working and will use the updated visuals after the mod update.