Its a combination of natural tire bloom and certain tire shine chemicals forcing the bloom to the surface. Bloom is normal, it is the process of the elasticizers, oils, and wax part of the compound that keeps the rubber flexible, prevents oxidation and UV damage, coming to the surface as part of tire use. Once it hits the air and sunlight, it turns that brown color. Some tires have more than others and can brown very quickly.
The secret is to CLEAN the rubber, ie; get deep down in the pores of the rubber and flush out the oxidation. There are many tire cleaners that will do this, however, you should avoid those with chlorine type bleach. It will strip away too much of those compounds and dry out the rubber. FAST. One product that I've used for several years with great results is Tuf-Shine cleaner. Depending on the level of bloom browning, you may have to do the apply/scrub/rinse a few times to get them properly clean. You can then use a dressing or leave them natural. Just avoid dressings with silicone.