Simply use a wash that's made up of a different kind of paint as the one(s) you have underneath... Most of my models are painted with enamels, so using a white-spirit based wash is risky... I therefore use water-based colours (the artist kind of stuff that comes in tubes and are to be found in artist/craft stores), mixed with water, a bit of white vinegar and a drop or two of dishwashing liquid. If you've overdone it, simply wash the wash away with water!