Retzuous must be black. This is a halachah l’Moshe m’sini (a law passed down from Moshe but not written in the Torah). Pictured above is a retzuah that is posel because of the brown line going from one side to the other. The person who was wearing them was completely unaware that his retzuous were posel. He may have been wearing them for years like this. He happens to be a well known rabbi and educator. Being that he is learned and knowledgeable about the halachos of tefillin, how could his retzuous get to such a state? Because the spot where it is brown was not visable to him unless he turned them over and inspected them. There is an easy solution to this common problem (besides checking your straps from time to time, which if you are like most people you’re probably not going to do). You should buy Ultra Double Black Retzuous. They are dyed before they are painted, so even if some of the paint comes off, they will still be black (and kosher).
Here is another picture of posel retzuous. They are posel because the paint has plastic in it and they are peeling. If there is plastic in the paint but they are not peeling, you do not have to worry, but if you see them peeling, they should be replaced.