To properly get rid of the brown parts on your tires, you need to first use a Wheel & Tire Cleaner such as our CarGuys Wheel Cleaner. The best way to do this would be to use a stiff bristle brush. I would suggest a stiff bristle brush like this:
https://www.amazon.com/gp/product/B000OCH8BI/ref=oh_aui_search_detailpage?ie=UTF8&psc=1

You spray the wheel & tire cleaner onto the tires, then since your tires are already brown, I would suggest doing at least 2 rounds of hard scrubbing/rinsing, spray the cleaner again, scrubbing/rinsing, with the stiff bristle brush. This should get rid of all the brown parts.

The next step would be to thoroughly dry your tires, then hit them with our Tire Shine to rejuvenate and keep them protected.