Answer is: A chemical change occured when a metal changed color because it reacted with another substance.
In chemical change new substances are formed, the atoms are rearranged and the reaction is followed by an energy change.
For example, reaction of magnesium (Mg) and oxygen: 2Mg + O₂ → 2MgO.
Magnesium change color from silvery-white to white in magnesium oxide (MgO).
