From a philosophical standpoint, I am okay with the rule as long as you have to have different colors on each side.
If ITTF requires black on one side and any color that is not black on the other side, there is part of this that could make sense. Because, as a player, you would always know that black is the other color when playing someone who twiddles with pips.
However, from a manufacturing standpoint, there is a problem. The black rubbers will move off the shelf because, 50% of the rubbers bought HAVE to be black. But the rainbow of other colors all competing to be the second color will end up having a longer shelf life. Because the other 50% of rubbers will all come from that rainbow of color choices.
So, no matter what you do, unless you get the rubbers straight off the press, from the factory, the way the pros do, the added colors will degrade on the shelf so the quality of the other colors, because of the oxidation that occurs to the rubber and sponge while sitting on the shelf in stores and warehouses.
==
Okay, so here is a story. I have a friend who bought T05 and T05fx for his racket within days of when I bought the same. I got the rubbers from a friend who was, at the time, sponsored by Butterfly. He ordered them and had them shipped from Japan. My friend got his rubbers from Paddle Palace. When I felt my racket with my new rubbers, I could tell they were pretty much fresh off the press. That was the best set of Tenergy I have felt to date. The sponge felt alive and fresh. The topsheet too; it felt grippy and responsive.
My friend's rubbers, felt how Tenergy had always felt before I tried these rubbers shipped straight from Japan. But compared to the rubbers on my blade, his rubbers felt older, not as alive, less grippy, the sponge felt less juicy, the topsheet felt less elastic. The comparison was pretty stark. And I could tell my friend was upset at how much worse his rubbers felt in comparison to mine.
I may never have noticed this without him wanting to compare setups when both had fresh rubber on them. But, it was obvious that the rubbers I got were far less oxidized and felt far better than the ones that had been sitting for a few extra weeks or months on the shelf at Paddle Palace.
==
Now, if people are consistently buying a black rubber and then, some random color, the people who are buying the same exact rubber (ie T05 on both sides) but different colors, they are going to notice how much worse the random colors feel than the black. And that will be because of oxidation during shelf life.