In summer we should wear cotton clothes, white and light coloured clothes because white and light colour clothes are poor absorber of heat. These colours reflect the heat away and keep our body cool whereas black and dark coloured clothes absorb and retain heat.
Cotton is also a good absorber of water. It absorbs sweat from our body and exposes the sweat to the atmosphere, making the evaporation faster. During this evaporation, particles on the surface of the sweat gain energy from our body surface, making the body cool.