Why can't Dye affinity reach 100%?

As the dyes not only dye the pulp fibers they also dye Anionic trash and fillers. The first pass retention on paper machines is never 100% and in some cases Filler retention can be as low as 50% so this colored mater stays in the backwater. When dyeing it is important to Close the system as much as possible to ensure little or no colored water is discharged into the environment.