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?

[GoldenHoops]

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.