No readabilitiy in the bottom two mockups. The top right one is excellent, though.
I like the use of thick brushes in your WIPs, reminds me of im9today.
Really, I think you just need to find a good way of separating background, foreground, and entities from eachother in those bottom two.
Agreed, the top left one looks great, but the character and enemies are somewhat lost in the bottom two. Darkening the background somewhat would probably help some.
One suggestion I would make for the bottom left one specifically: At first glance I thought the background was actually snow-covered mountain peaks, rather than a clear sky with white clouds. You could change the sky to a slightly darker mountain background in the middle third of the screen (where the characters and enemies are going to be mostly), with the tops being just about where the clouds are now, and then keep the current lighter colour above them to show the sky. You would have some layered colour while at the same time it wouldn't be as dark as the cave environment in the top left one, preserving the distinction between the two. The ground blocks in the bottom-left one could also use a colour tweak to differentiate them from the background
For the last one, where it looks like the protagonist is flying, perhaps you could make it night time, thereby also warranting a slightly darker background?
I do love the art style though, and the enemies remind me of classics like Ghosts 'n' Goblins, Bonk, or... Mac and Joe: Caveman Ninja (I think that's what it was called).
Developer
