The sidewall of your tyres turning brown is a common phenomenon called blooming, where antiozonant chemicals that are added to the tyre during manufacturer to prevent cracking and browning from exposure to UV rays come to the surface, giving the brown appearance. You may not be able to remove the brown discolouration entirely, but using a good quality wheel and tyre cleaner or degreaser will help remove as much as possible, followed by a tyre gel which will help restore the black finish.