[Psientist]

well, as long as the sprites have been properly alpha masked, the only difference the background would make is what the foreground edges were antialiased against (and even then, not so much depending on how the masking was done). I like to use grey as a neutral background so that it gives minimal artifacts against either white or black or color terrain backgrounds, but I guess it's up to the players to let me know if my theory worked.

As to the shadows... I purposely left out the ground shadows on a couple of units, but I might have just missed some by error I guess. My default method for the shadows is to not have any foreground color for them, but just a 50% alpha mask where the shadow is supposed to be, so that whatever the terrain background is gets partially darkened. Again, that was my theory, it seemed to work out in the few tests I specifically looked for it. Let me know if not though.