Here #2 and smaller have to be white.
I would say this requirement is out there because those sizes are manufactured white. The bigger ones are not so they allow you to colour tape them.
If people were to just use black all the time and tape it white, I bet that some might do a crappy tape job and it would fall off, some might not colour it at all, there would be confusion on what one is the identified conductor.
Your supply place is just trying to make more money by only selling by the roll.
