Anthocyanins as pH-indicators: stop the titration at red, purple or blue?
If you titrate some hydrogen bromide solution to some sodium hydroxide [...]


It's probably a bad idea to start with the anthocyane in strongly alkaline solution. Like other pyrylium compounds, flavylium cations do not tolerate (strongly) akaline conditions very well.

The nucleophilic opening of the pyrylium ring to a chalcone is irreversible under the conditions of the titration.