One side of your bat must be black, the other clearly distinct from it — classically red. Over 90% of pros run inverted, the smooth, grippy standard rubber, on at least one side.
The colour rule isn't cosmetic: the two sides are allowed to behave completely differently, and the colour is your only clue as to which one just hit the ball. The standard is inverted — pimples facing inward, smooth surface outward — because it generates the most spin; more than 90 percent of professionals run it on at least one side. The alternatives are short pips, long pips and anti. Rubbers have to be ITTF-approved and meet a minimum friction requirement, and the blade itself must be flat and rigid. The expensive beginner mistake sits on the DIY-store shelf: hard, glossy cheap rubbers barely take spin and slow your learning down. What often goes unnoticed: the colour tells you the side, not the type of rubber. Play someone with pips for the first time at a club and you'll notice exactly that — you have to read the spin afresh instead of answering out of habit.