I put a purple collar on Cally (my dog with palest coat) and sprayed detangling solution with silicones and ethanol in it on her to make sure the leather didn't react with it and the colour didn't come off on her. Neither of those things happened, although if you are going to use detangling spray or any other grooming product on your dog, I would recommend taking its collar off first and allowing the coat to dry before putting it back on!
Adhara and Hobsey have been wearing collars at agility and obedience respectively (both of them to pull and be silly there because of other dogs and people) and going for lead walks in them. Neither of them have knots in their neck hair.
Adhara decided to make hers dirty by running with Indi into a filthy field that is planted with grass seed after rain had saturated it. This is her collar before and after cleaning with saddle soap.