Done some messing about. This is what it looks like. Color switching is a bit of an issue. If you leave in the original grey it acts as transparent, so when your engine does the coloring it will bleed through. I threw together 'ye old grass tile' common in old school RPG's. Throw on a different color and it becomes road, grass, dirt, blood, etc...
If you wanted someone to do a full tile set, it'd probably be better if each unique object, enemy and terrain feature called to a unique sprite. That way you can edit them individually and not play transparency tricks, you know?
The transparency thing makes a good exercise in limitations though. Like the spider sprite is going to be a transparent body (later colored by your game engine) with huge red spidery eyes peaking out. Or at least that's the plan.
No transparency tricks. Pretty clean.
Some tricks. Multi-use grass tile.
Many tricks. Shrub and Tree use grass tile type things in the background...too busy?
Note that making the backgrounds transparent for the wolves and player doesn't work. The backgrounds are always grey, they don't take on the color of the underlying terrain. :-(
EDIT: Hey NON, it looks like your game is drawing not just the black part, but rather it lays down a background color then draws EVERYTHING BUT the grey color of your original sprites. So when spriting anything you want to switch colors needs to be that grey/transparent color.