(Sarah of the Clue Crew gives the clue.) We've rubbed the pigment of a radish on these cards; lemon juice turns the pigment orange, indicating that it's an acid; milk of magnesia turns it blue, indicating that it's this
The colour in red cabbage (it is a pigment called an anthocyanin) makes 
a... [DOC] [TLE] 4.9A Natural Acid/Base Indicators: Red Cabbage Juice Extract [PAR] 4.9A Natural Acid/Base Indicators: Red Cabbage Juice Extract ... some foods and 
household chemicals is illustrated using red cabbage juice indicator.