I know this thread has been dead for a while but I really wanted to chime in on the topic.
Bluzman is right with his recommendation for Wesley's Bleche White. I've been using it for over 30-years with great success. White walls, white letters, black walls - it works great.
With the tires dry - spray on the Bleche White. Using a wet brush, scrub the rubber. Rinse it off. You'll be surprised how much crap will wash off! It leaves behind perfectly clean rubber. I caution you though - make sure you rinse the rims off very well or it will leave spots.
I generally don't use a tire dressing for two reasons; dressing seems to be a great dirt magnet and using Bleche White the tires look great on their own.