Chances are the stain has set if you already ran it through the washer/dryer. You can try different things like using vinegar as a stain treatment, or Avon bubble bath... Or any other DIY stain treatment (google it and you'll find a list. I find vinegar is one of the better ones, though. Avon bubble bath lifts stains before they set for the most part)
You could always take it to a dry-cleaner or buy a store bought treatment.
Those paints are water-colour paints. If it happens again, keep the stain from any heat and try treating it and washing by hand in luke-warm to cold water. Heat will set it.
