Why does tea change colour when lemon is added to it?:confused:

Lemon juice contains citric acid which largely prevents the ionization of the hydroxyl groups of the tea polyphenols.
The spectrum of the more acidic lemon tea therefore differs from that of plain black tea, and the beverage color appears light yellow rather than the usual reddish brown.

More here: Stash Tea: Tea Times - Milk Or Lemon For Your Tea?