For showing, is it customary to use white pads for dark colored horses and black pads for grays? Or are both white and black pads ok regardless of the horse's color?
Showing in what? If you're doing the hunters or the equitation, you should only use a white fitted pad.
If you're doing the jumpers, you can use whatever you want (except for classics, then white is most appropriate), but I personally think white looks classiest.
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Exactly what Supershorty said.
The reason that people you black pads on greys is that no matter how hard you try you can't make your white horse as white as your white pad... otherwise what Supershorty said.