Overworld solidity is controlled by tile, so changing one of a certain tile changes the rest.
Ethereal: You can walk on it.
Solid to PCs: You can't walk, but you can fly.
Solid to Hovercraft: Leftover from early design. Ends up being no different than Solid to PCs.
Solid to Dactyls: Dactyls can't fly over it.
Exit: You can walk and enter a different area.
Try changing the deep ocean tiles to exit, and you'll have an ocean of yellow.