Why are umbrellas mostly black in color?

Basically, the umbrella was initially meant for rainy seasons. In the olden days umbrellas were made of thick cotton cloth. When they got wet in the rains, they would take a long time to dry up. It was found that umbrellas made of black cloth dried faster than those made of cloth of other colors. So it became a standard practice to use black cloth in the making of umbrellas. Black cloth dries faster because black or dark surfaces absorb more heat from the surroundings than lighter surfaces and as a result the evaporation process is faster.
Today, umbrellas are made of fabrics that do not soak water, and so, as drying is no longer a problem, umbrellas come in all shades and colors.