why black colour umbrella is used in summer than a white one
the black umbrella absorbs all the radiation from the sun and gets heated up in this process and emits the whole black body spectrum, which is mainly heat (infra-red radiations).
So you could think that a black umbrella might be good for UV protection, but not at all useful for heat protection.
black umbrella is also in contact with the air, which cools the umbrella through convection heat transfer. This leads to a lower level of emitted radiation