Oxygen and Chlorine Bleaches
Cancel each other out to an extent. Looked up and posted the reason why and how this comes about once or twice before.
As one has stated often this is why Cheer powdered detergent boasted of "Colour Guard" formula; it contained small amounts of sodium perborate. Not in strong enough dosage to bleach but just the proper amount to help keep chlorinated tap water from fading coloured laundry with repeated washing.
As with most other things the reason is out there on the Internet. Any good search engine with the proper keywords should get you there.