Why is it better to wear a white T- shirt than a dark blue T- shirt in the summer?

It all depends upon what you mean by better.

A white t shirt will keep the wearer cooler because white reflects light, whereas darker colors absorb light. Since light is a source of energy, the absorption of the light energy will increase the temperature of the clothing. During the hot summer months most people want to remain cool, so a white t-shirt will be cooler.

Fashion has followed function in this area and generally white clothing is preferred during the summer. Darker clothing is typically worn most during the fall and winter seasons because it is generally warmer, since it absorbs light rays, and hence absorbs the heat of the light rays.