This can be used to help identify the metal ion in an unknown sample.
Aluminium (Al), calcium (Ca), and Magnesium (Mg) produce a white precipitate, and iron (II) and iron (III) form green and brown precipitates respectively.
Copper (Cu) ions produce a blue precipitate, zinc (Zn) - a white precipitate, and lead (Pb) - a white precipitate which dissolves in excess NaOH.