That looks really amazing actually. I do need to work on my palette skills; perhaps you can give me a piece of your expertise =O?
I just gave you some - as has Shiek, Shane, and even Moosh threw in a bit. As Shiek said, you need to always keep your environment in mind - particularly your light sources. What colour is the light? What direction is it coming from? Your highlights should be influenced by your primary light source, e.g. in sunlight your highlights should be skewed toward a pale yellow. Conversely, your shadows should be influenced by ambient light (not hugely important in bright, outdoor environments, but definitely in dark caves/dungeons). Shadows should also be indigo/violet shifted, because when light is obstructed the high frequency red and yellow waves are lost before the more penetrating, lower frequency blues (which is why you see very few reds when you start going deeper underwater). This is also why you can only really see blue at night time. Speaking of night time, our eyes have two kinds of cells - cones and rods. Cones are for seeing colour, and require bright light, while rods are used for seeing tone, and can work even in poor light. This is the other reason (main reason, actually) why our eyes only work in monochrome in the dark - we can make out shapes and shadows and things easily, but everything looks blue/grey because only the rods are working. I think dogs lack cones, which is how we know they're colourblind.
Anyway, this tutorial is one I always link to. It's a great introduction to colour picking and gives you immediate results.
What about this
I combined the dirt/treestump/mountain colors, and shared the lightest green for the trees/bushes and grass, and the darkest universal color is now the outline of the bush and tree? (Not in ZC yet, this is MS Paint work.)
Looks good, although since it's MS Paint I'm assuming the picture is 24/32-bit so I won't be able to check your palette. DO NOT USE MS PAINT FOR PIXEL ART! Seriously, 24/32-bit bitmaps aren't your friend. Grab Graphics Gale (the free version is fine for ZC stuff, but the commercial version is better), and only work in 4/8-bit mode (16/256 colour, respectively). I recommend working in 256-colour mode even if you're doing a 16-colour sprite, as it gives you space to arrange your palette better (as opposed to a straight line). You can then reduce the image to 16-colour and reorder your palette how you want it (hold shift and drag the slots around). This makes it more like ZC, where if you want to change a colour you don't have to make a new colour then manually recolour every pixel in the image (like you'd have to in MS Paint), you just change the palette and it's applied to the whole image.