Butterfly pea flowers have anthocyanins, which, as I learned when I wrote this post for a rainbow poke bowl with red (purple) cabbage, change colors when you vary the pH. At higher (more basic) pH, these compounds produce a blue or purple color. At a lower (more acidic) pH, they are more red. So adding lemon juice to Empress 1908 turns it pink because lemon juice has a very low pH (2.0) due to the citric acid. Tonic water, which is a sweetened combination of citric acid and quinine, also has a similar result and similarly low pH (2.5).