Black Clothes Turning Brown After Washing?

My son has a reversible cotton sweat top. It is black on one side, bright pink on the other with a white collar. I use softened water (we live in a very hard water area ) with my washing machine, Fairy Non-bio detergent and usually line dry or occasionally tumble dry. The problem is that the black is turning brown, but the pink is fine.

Given that I only ever wash this top with dark clothes (not jeans) how can I stop this from happening? Other tops within the club have gone grey or stayed black. My son's is the only one that has gone brown. Any idea why or how I prevent this happening? Thanks.

By Jillibean