The brownish color on the sidewall of your tires is not a defect. The source of this discoloration can be varied. One possibility is that the tires contain an anti-ozone agent in their rubber compounds to slow down the ill effects of exposure to ozone in the air. This anti-ozone ingredient will migrate to the surface of the rubber and leave the appearance of a brownish dust. This is completely normal and technically is no cause for concern. In time, depending on usage, it will disappear.
Other possibilities for discoloration can be simply dust that is picked up from normal driving or brake dust which is generated by the abrasion of the brake pads against the brake rotor. This latter condition is more prevalent when the brakes are new or have recently been relined. In all cases, we recommend that you continue to clean your tires with a mild soap and water.