I've only tried this on Louis Vuitton bags, so not sure about others. Got marks on my bag's handles and decided to do what I always do to them.. rub them off. Just sharing a tip.
Take a CLEAN rubber [preferably new] and just rub the dirt/discolouration. This is usually due to it rubbing against things like denim just like a sofa would discolour too.
That's all for now. Blog was a bit inactive so just updating with random little posts.