What makes a bathing suit chlorine resistent?

Answer 1

Not all bathing suits are chlorine resistant. Older, all cotton suits which were dyed with natural colors tend to fade quickly. The chlorine reacts with the cloth and/or dye to cause this fading.

Newer, chlorine resistant clothing, including bathing suits, are usually synthetic polymers which have been colored with special highly stable dyes or even mineral pigments. The bonds in these materials are so strong that the chlorine has a hard time reacting.