Why did the solution turn green?
Both citrulline and arginine are nitrogen-rich, basic amino acids. A basic acid sounds confusing, but that's the way it is.

The colour change indicates a change in the pH of the solution (the tea mix).

The pH indicator that changes the colour is in the hibiscus flower and belongs to the class of the anthocyanes.

Under acidic conditions, the dye exists as a deeply-red flavylium cation. At a slightly higher pH, the dye is reversibly converted to a colourless form. Here, the colour of the mixture is determined by the other ingredients. When more base (read: more amino acid) comes to action, further colour changes are conceivable and the double deprotonated form of some anthocyanes are known to be bluish-green.