I totally agree with Vort (page 2). I have had garments stained by all sort of stuff you can imagine, since I love cars, food, outdoors, dogs, little children, and used to love my ex wife XD.
Getting clothes stained is just part of life, and that includes underwear, so I think it's a good idea to have a "paintballing garment". But if you still feel uncomfortable ¿why don't you try airsoft? This allows you to play war without getting your garments stained.