Which side of the ping pong racket has more spin red or black?

Competitive players choose their rubber sheets carefully, and often have specific preferences for the red side and the black side, and they are usually not the same. An offensive rubber is usually more grippy to impart more spin, and is often faster (more bouncy, like a hard trampoline effect). The other side is often more defensive, sometimes with less friction so the opponents spin is less effective. This varies greatly from one player to the next, but it is somewhat more common for the black side to be more grippy (and spinny). Some of the lowest friction rubbers are only available in red (some of the Dr. Neubauer rubber, for example). Many also believe that the black version of many of the grippy rubbers are more grippy than the red. So, yes, it depends, but there is a slightly better chance that for any given high-level player, the black side produces more spin.

It depends. The most important point is that the other player has the right to check which side has more spin.

1) "Table tennis regulations allow different surfaces on each side of the racket. The different types of surfaces provide various levels of spin or speed, or in some cases, nullify spin. For example, a player may have a rubber that provides much spin on one side of his racket, and no spin on the other side of the racket. By flipping the racket in play, different types of returns are possible. To help a player distinguish between different types of rubber used by his opposing player, international rules specify that one side must be red while the other side must be black. The player has the right to inspect his opponent's racket before a match to see the type of rubber used and what colour it is. Despite high speed play and rapid exchanges, a player can see clearly what side of the racket was used to hit the ball."
Source and further information:
http://en.wikipedia.org/wiki/Table_tennis#Racket


2) "Since 1988, it has been an ITTF and USATT rule that rubber must be red on one side and black on the other. This came about because of the rise in the use of "junk rubber" (the name some frustrated players call long pips and anti-topspin rubber). Players who use vastly different sheets of rubber on each side of their paddle had perfected the art of disguising which rubber sheet they used to strike the ball, and consequently what spin was on the ball. The rule change has made it easier for spectators and opposing players to understand the spin that is happening during table tennis play."
Source:
http://www.spintechnologies.com/tech.htm