Yes, if you are swimming in a pool that has chlorine in it. Some
other chemicals added to swimming pools to clean the water react
with your hair. It can turn blond hair green and it can change the
color if you have dyed or rinsed your hair color.
I think you should dye your hair after swimming because the hair
salon would probably wash your hair for you if you ask them ($8)
and when your hair is not bleached again (water in a swimming pool)
after you dye it, it should hold up much better.