the path needs to show more depth, lighten the front rocks, darken the back rocks etc
It is not actually a light or dark issue in a matter that you just increase or decrease the light, that's why you are having depth problems. You need to work more with contrast and saturation. More contrast, saturation in the front, and gradually less in the background. If you have a look at the horizon. Things closer to your eye is easier to see and there won't be enough particles ect in front to dissort(Not brighter) and things far away is brighter because there will be particles off all sorts in the distant between your eye and the object far away that you look at. Those particles also make the background seem "less saturated". Nothing is invisible, like air and moist. They are just very small bits, but clumped together they will create a somewhat transparent layer.
Same effect will be seen in water, there is a limited distant that your eye can see that differs due to the ammount of particles in the water.
When it comes to games and such you do it because you want the eye to focus on a specific depth. And since our brain somehow is drawn to the sharper and darker parts you make the foreground darker. You don't want the players eye to focus on the background, it's just there to give you the feeling of room and depth. That's also why you don't want too much details in the background, at least not details that draws focus in a way that distrubs the player and confuses the eye. Like in your case, when I look at the mockup my eye is drawn to the big dark piece of open space in the background.
I did this tileset very quickly but I hope it helps you:
There are other ways to do this like
Candy Man Criminal might have been aiming for.
Like from
Cave story
But then again it is a matter of saturation and contrast. More contrast and details in the foreground objects and tiles and in this particular case nothing but a flat colour in the background
