Frustrating bug is frustrating.
Modified demons are a special case, they're the only thing to take a pre-existing sprite and apply a color filter to it, so it makes sense to me that something could go wrong with them, but not with any other art.
But what I can't begin to guess is why this would be happening just for you?
At the risk of sounding like a crappy support desk cliche, do you have any video drivers you can update? I will see if there might be some sort of obscure error in my shader code, for my part. But I will confess I'm pretty baffled here.