Help!... My white trouser was stained with yellow-colored pants?

What I would do is use Oxy Magic Stain Remover (it comes in a spray bottle) as well as using some OxyClean in the wash and since they are white, I might be tempted to use a wee bit of bleach. Wash them in COLD water (hot sets stains). I have used the Oxy Magic spray for several years now and it has gotten out many set-in stains. Good luck.