Just be careful with the strong cleaners. That brown might possibly be part of the normal UV protection of the tire. A strong degreaser might actually cause the tire to brown even more after a few heat cycles.
Do a Google search on "tire blooming".
I typically wash the tires with the same wash product I use on the rest of the bike. I also use a good firm brush.
I like
Meguiars Gold Class Car Wash and use it on the cars and the bikes. It might take more elbow grease than spraying a strong chemical on the tire but I have never had a problem getting my tires clean this way.
One the principles you will hear mentioned here and one that I always practice in detailing is to use the least aggressive product possible that will get the job done.
You don't want to go straight for a sledge hammer when a rubber mallet might just do the trick
Here is a pic of my 1997 Honda VFR 750
