There are certain types of players who use very different kinds of rubbers on each side. And with these rubbers they will switch which rubber they are using in their FH or BH in the middle of a rally. The act of switching sides is called twiddling. When a player twiddles with skill they can use one rubber with one effect on FH on one shot and the other rubber with a very different effect on the FH on the next shot in the same rally.
The reason that there is a rule that you need one red and one black rubber is that players used to use two black rubbers and twiddle and you couldn't figure out what kind of shot was coming at you. Now, if you watch the player's racket and pay attention, when someone twiddles and is using a special rubber like anti-spin or pips on one side, you just have to pay attention.
So yeah, red can be either FH or BH. And you could even change which in the middle of the rally.
So, choose which one you want on FH. But you have to have one that is red and one that is black.
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