Removing tan marks from unwashable/non dry clean white fabric?

[endedans]

I need to try to remove, or improve/cover up some tan marks from an item of clothing (white!) which can't be dry cleaned or washed (think costume).
Any tips appreciated.

No idea how the person who told me this found it out but smokers toothpaste removes tan. The tooth paste is blue so mix with a little bit of water before using on an old toothbrush, then wipe off with a damp cloth. (Might even be worth trying regular toothpaste but suspect the smokers stuff has whitener it it)