I just want to say that I've been bathing my puppy in the Isle of Dogs Evening Primrose Oil Puppy Shampoo and I LOVE IT. He's so soft and silky. There are so many people here with new puppies, I just wanted to share.
OOOOps sorry, so unlike me, LOL it was a typo error, what it should of said is which CAN, not which can NOT.. Drop the Not..Dolce/Louis:
Now that you mentioned Ile of Dogs and your black boy turning reddish, I do recall someone at the shows in past mentioning that something in the IOD shampoo makes their dog's black coat turn rusty in color.. so maybe you should only do the silver with IOD and use another shampoo on your black.
I would not use any color enhancing shampoos unless your dog is showing. I believe that any shampoo that has coloring agents used to enhance the white or black coats contains chemicals which can not be harmful to your dog with prolonged use.If it is a pet, I would not care if the coat tips are reddish, most likely due to sun (since you said he is black and not blue) and I would just leave it at that. I would prefer a shampoo that is natural and one which does not contain color enhancers.
For pets I would use and like Desert Almond by Groomer's Edge. It is a basic shampoo smells good and is great for the dogs. They also have good conditioners.