My grandmother is 91 years old and she spends her days knitting dishclothes. I am younger than that and spend my days using dishclothes. I wash them but after a few weeks even with regular washes they start to smell and that is when I make Dishcloth stew.
I put some water in a pan, add some bleach and the dishclothes and boil them for a few minutes. The dishclothes do get a bit lighter in colour but they smell better after that. To me smell is more important than bright colour.
Some of my stash. Usually when I see my grandmother I give her some more yarn and then in a few days I get some more dishclothes.