Potato

I know how to fix it, but it is a bit hard on fabric... We have a product here in Australia called 'Exit Mould'. It is used to clean really bad bathroom stuff. So you can find a similar product over there, the ingredients are; Sodium Hydroxide 2.4g/L Alkaline Salts 1.2g/L Sodium Hypochlorite 42g/L. It is not pure bleach, but a mix.

I have used it to remove small colour runs by dabbing the run with a cotton bud dipped in Exit Mould so the cotton but is fairly damp but not really wet. A bit tedious, but it got rid of the colour and didn't yellow the fabric. Obviously you need to be careful not to let it get on the colour you want to keep! Once you set the remaining colour with an iron, wash the fabric to remove the Exit Mould.

This same stuff does other amazing things - it will take a coffee stain off a painted wall without changing the paint colour in any way. Got brown stained fingers/fingernails from beetroot, plums, assorted plant material, hair colour? Little squirt onto your hands and it vanishes instantly. (Then wash your hands really well and use some hand-cream...)

This will only work on cotton fabric if you want to return the cotton to white!