If it's so easy to wash grey INTO a bra, why won't it wash out again?

[BreakfastAtSquiffanys]

Just that,
I'm really careful to segregate my colours and whites doing laundry but must have left a black sock in machine and my fave white bra (which was still white after many washes) went grey.
So why won't it won't it wash out again?

Very irritating and it's a ploy to make us buy more white bras. Try milton or glo white to get rid of the grey.