Well, this is a big problem for my little girl too.
She's puppy pad trained & it seems to soak into the pad and onto her back legs. She squats really low. Her legs are very light colored. I've tried everything, but recently read about using Woolite. I was very apprehensive about trying, but it did seem to improve with helping to remove the stains, which other shampoos had not. Wish I had a solution about the problem itself.