Just as there is a build up of uremic toxins in the blood and tissues of people with kidney failure, there is also a build up in the skin. In this case, the particular “offender” that used to be referred to was called urochrome, but it’s probably more complex than just one toxin.
When folks get transplants–or do daily or nocturnal dialysis and get a LOT more treatment–their skin color returns to normal. So, part of the problem is likely that standard in-center treatment is not removing enough toxins.