The correct answer is - chemical weathering.
The deserts are in general covered with sands. These sands are the result of weathering, mostly caused by the mechanical weathering of the wind. With the process of weathering of some of the minerals are created another minerals, like the iron oxide, which is possible only if the weathered minerals contained iron of course. Because the iron naturally has a reddish color, as the sends accumulate more and more, the desert starts to get that distinctive reddish coloration of its landscape.