One side of an approved table tennis racket must be black.
Under modern equipment rules, the two racket faces must be clearly different colors so the receiver can distinguish the surfaces used to strike the ball. The other side is commonly red, although current regulations describe it as a color clearly distinguishable from black and from the ball.
The rule addresses a tactical problem: different rubber surfaces can produce different speed and spin. Without visible color contrast, a player could turn the racket during a rally and make the change difficult for an opponent to detect.
The black side is therefore not simply a traditional decoration. It is part of an equipment-identification rule. Players must also use approved coverings, and officials can inspect rackets before or during matches.