There are several reasons. One is that white fabric is transparent, and the sun will reach the skin. Black fabric blocks the sun, and the heat is kept in the up

There are several reasons. One is that white fabric is transparent, and the sun will reach the skin. Black fabric blocks the sun, and the heat is kept in the upper robe, and doesn't penetrate inward. Another reason is the structure of the garment - it's a robe that is open from top to bottom, this allows air currents to flow along the skin, drawing away moisture and cooling the person. The heat of the sun acts as the engine that causes that airflow (i.e. it's worth slightly higher temperature, which will then cause the hotter air to rise up, and out the neck carrying moisture with it). And finally, you don't want a sunburn. Modern heat control is all about exposing the skin for the cooling, but it relies on shelter and sunscreen to block the sun. If you can't block the sun then you want a garment to do it.