Definition: In table tennis the two color rule is used to ensure that the opponent has a fair chance of telling which side of the bat has been used to hit the ball. Before the two color rule was introduced, any color could be used for table tennis rubbers, and players were using combination bats with the same color on both sides and twiddling the bat, making it very difficult to tell which side was hitting the ball.
This was considered to produce table tennis which was not spectator friendly, since it was difficult for uninformed spectators to understand why players were making seemingly simple mistakes.
The rule as it now stands states that the surface of the covering material on a side of the blade, or of a side of the blade if it is left uncovered, shall be matt, bright red on one side of the blade and black on the other.