I don’t personally own a Belt Bag (would love to), but I can tell you that all my light colored (white, beige, pink, even red) bags and shoes I own have inevitably suffered from color transfer (especially from denim) regardless of the brand and leather type. By the way, I read that it helps to use a damp white cloth and rub it against your jeans, but I haven’t tried it.
Sometimes some alcohol free baby wipes have done the job, sometimes not, but I may try someday some of the products the ladies have recommended over here. There are many useful threads covering this issue and I’m sure they’ll be much more helpful than my personal advice.
Overall I think it’s just part of having a light colored bag and I’ll just try to keep them as clean as possible and wear them on the same side always. Hope that helps a little!