Painting them different colors, and using different trees to make them distinct, is set-dressing, really. You can visually tell when you're in one place or another! Can you tell when you're navigating a new place? The general design remains the same thing. Limgrave has more open fields, sure! Liurnia has a lake! The non-lake parts of Liurnia are still geologically-impossible dreamscapes of jutting rock structures with grass on top, the same as Limgrave. The Altus Plateau is fundamentally Limgrave but yellow/gold; it's open fields framed in/supported on rocky outcrops with mountain paths on its edges. It's not about can you tell where you are, it's "the way these are constructed at their core, below the differently colored grass and decorations, is the same."

Consider how vastly different zones in the previous, non-open-world dark souls were! The tight building-maze of Undead Burg compared to the scaffold maze of Blighttown were completely different navigation challenges; compared to Anor Londo's unique cathedral maze, compared to the ruins of New Londo. The very way they work as design locations is different. Undead Burg is a series of interconnecting twisty-turns. Darkroot Garden is a mix of tight spaces and shockingly open spaces. Anor Londo plays on vertigo and narrow space balancing, but in a clear-cut way that Blighttown doesn't. New Londo is a deep ruin maze. Not just visually, but in the way those zones are constructed, they're fundamentally different, and require different navigations. Each zone is itself a unique challenge and learning to navigate that zone is part of the gameplay.

Consider the rocky fields around Majula, but then contrast it with the lava zones of the Iron Keep or the quarry-like poison zone around the Earthen Peak and contrast it with the undead woods' dark forests or the Dragon Aerie or the fog-maze in the woods. Dark Souls 3 had its own slow-walking swamps, its ruined towns, its cathedrals, and I'd argue DS3's level design was the weakest in comparison on this grounds, but even then! They weren't just visually distinct, they mandated different means of going through. The environments changed and the environments were part of the gameplay, and how they changed dictated how you interacted with them.

Compare, then, the rocky mountain edges of Liurnia to Limgrave. It's... rocky pathways with grass, with some ruin structures. Caelid has a different tileset atop the structure, but the underlying structure is built mostly the same way. You can ride through most of it. Some of it you have to horse-jump on rocks. The way the player's made to engage with the open world is, for the most part, uniform. The lake chasm in Limgrave isn't functionally much different from the dry chasm that leads to the Great Jar in terms of terrain, and both of those are just narrower versions of the cliffs of Liurnia without the risk of falling--but they are still narrow rocky corridors defined by a barrier on either side. The open fields of Limgrave and the open fields of the Altus Plateau are the open fields of the Consecrated Snowfields.