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.
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Some tricks. Multi-use grass tile.
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Many tricks. Shrub and Tree use grass tile type things in the background...too busy?
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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.