Why are umbrellas mostly black in color?

It's just because black colour has an advanced ability to absorb heat radiations much better and faster than any other colour. Then you might ask why does then we use it at sunshine to prevent body from heat. No surprise to your question it's because at the same time black is able to liberate back the absorbed heat at a faster rate. If you doubts this then you doubt about one more thing that why does most of the umbrellas are made silverish inside it. The answer to this is that black expells heat in all directions so in order to prevent heat entering inside the umbrella it is painted silverish inside as silver acts as a mirror, so it just reflects the heat back out to the umbrella. So we finds much cool inside with a silver coated umbrella than a black one.