Taste, colour and mouth feel depend on the interaction between the two main components of tea, polyphenols and caffeine. Each component is astringent on its own, but as a complex the astringent character is reduced. Water is known to contain dissolved gases absorbed from the air.
Carbon dioxide (CO2) gas that is present in water affects the acidity. Acidity of water plays a critical roll in the ionisation of tea polyphenols and it contributes to the stability of the above complex.
CO2 in water is gradually released during the boiling process. Re-boiling will in fact further reduce CO2 levels, resulting in a decrease in the acidity. As mentioned above this will affect the caffeine and polyphenol complexion, and bring about changes in the colour as well as the character of the brew.