That's really a rather funny idea, and I think the shading is pretty good... nice depth in the picture, in my opinion :)

It doesn't blend too well to the green bars and the bottom left corner, though. I've only done a few tiles myself, but I've found it very helpful to zoom in and use the colorpicker and then a small (usually soft) brush to get as close as I can to the edge(s) I'm given. This is then repeated all the way along the edge(s). If this gives sharp edges inside the tile _careful_ use of the smudge tool with a small, "dotted" brush can take care of that.
Also, I start my coloring at the edges to make sure I don't have to brigde large gaps of color, if you know what I mean... you can't smudge away a large difference in color.

Oh, and I should mention that I work on a copy that I've scaled up to 640x640 (as opposed to the 160x160 we're given) and then when finished I scale it back down and copy my part of the picture into the original image. This makes it easier to work on details... If you scale up your image, you should choose a size that is divisible by 160, otherwise you might get rugged edges when you scale back down.

I hope I made sense and that some of it can be of use.