Tbf, I think overall color is the biggest thing to get right on lands, but yeah, it's always nice when the subject matches too. My guess is they're features in the domains of the various colored factions from the set.

They have beautiful art but my first guess was swamp for the plains art.

Agreed, but I still love it!

The others tho, let us analyze. The plains does look almost exactly like the swamp, only difference is that it is less humid. The artwork does not show any plains at all. I also would assume it was a swamp from the artwork. The island is no island, but this is already a tradition now. It is a river in a valley between trees. With other colors this could be as well a forest or mountain. The swamp is alright, like I imagine a swamp where some witches live. If it was greener, it could be a mangrove forest or a bayou as well. The mountain shows a valley with some ladders. Well I guess the valley is between two mountains, so I will let it pass, even if the valley is the absence of mountains. Lastly, the forest. You see a bush tunnel, and behind you can see exactly one tree. You have more trees on the island, swamp and planes artwork. I have one tree on my balcony, ai guess it is a forest now as well. I stand by my point tho that it will look nicely foil, even if someone downvoted me for this opinion :)