Personally, the white tape I have seen is textured, and a bit rough on the backside of the thumb. If using white, I prefer it on the front of the thumbhole. Black tape is ok on the back, since it is generally smoother and untextured. But like I said, that's just my preference. It all depends on what feel you like and what gives you the grip you need without causing any hang ups or dropped balls.
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Ain't Skeered!