(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
[DOC] [TLE] Science Club - an acid-base indicator using red cabbage - WebInnate [PAR] The indicator will change colour depending on whether an acid or an alkali is 
added.