A real table tennis bat must have a red rubber on one side and a black on the other to be used legally in competition. What is the reason for this, and doesn't it matter what you use in your advance or backhand?
Why red on one side and black on the other?
It was many years ago that the ITTF introduced the rule that one side of the bat should be covered with red rubber and the other side black. The main reason for this was that many players used different rubbers on either side of the bat, some dramatically different, such as anti-spin on one side and a fast spinning rubber on the other. The idea was that opponents needed to be able to predict what was on the incoming ball of the opponent's battle stroke, and it should not be down to guess which rubber they used. With the two gums, different colors gave the opponents the opportunity to see which rubber was used for which stroke.
These days are a common question why some players, or even whole teams, always use a color gum on the front and the other on the back. Are red and black rubbers, even identical brand and type, in themselves different?
Well consensus is that yes it is definitely different, but for some rubbers it is much more important than others. For Chinese sticky style gums, the difference is usually most obvious; The red rubber is a little less sticky, and the black is a little softer and sticky. The sticky surface slows the ball down a little, so it makes the red a little faster than black. So you should choose the one that suits you better in advance. For a more spin-based looping game, a black rubber can be a better choice to help generate spin. For more of a stroke / driving style, the red may be a better option, which not only gives you a little extra speed, but the red rubber is also a little less sensitive to incoming spin, making it more forgiving.
This difference is believed to come from the manufacturing process. The raw rubber used to make the rubber tops is naturally sticky and black. The color needed to make the rubber red, it loses some of its sticky properties and softness. For some rubbers, the different properties are quite obvious. For example, a black "729 Geospin Tacky", one of the spinnable gums manufactured by the Chinese manufacturer "729 Friendship", is a pretty much sticky in black than the same rubber in red. Because players usually buy such a rubber because it is the great spinning potential, the black is much more popular than the red one.
For many Japanese or European made rubbers, most of which are in themselves non-sticky (but grippy), the differences are not noticeable so the decision is not as important.
So the conclusion is that the decision on which color you use on which page really makes a difference, although many players are not even aware of this, and their decision is based on personal preferences or simply inherited from other players. For a more spin-based game I would recommend black on which page is used more to generate spins. For a more speed-based game, red on your attacking page may be more appropriate.