The two different coloured Co(II) complex ions, [Co(H2O)6]2+ and [CoCl4]2-, exist together in equilibrium in solution in the presence of chloride ions:
[Co(H2O)6]2+(aq)(pink) + 4Cl-(aq) ⇌ [CoCl4]2-(aq)(blue) + 6H2O(l)
This equilibrium can be disturbed by changing the chloride ion concentration or by changing the temperature. The colour changes accompanying the changes in equilibrium position are as predicted by Le Chatelier’s principle.