Why are transition metals coloured?
To understand this fully is difficult. When a central Transition metal ion is surrounded by ligands, (that is groups of ions, or molecules that attach themselves to the central ion) the `d` electrons on the central ion can be promoted to a higher energy level. When it falls back to ground state it gives off light in the coloured range of the Electromagnetic spectrum. Exactly why this happens or predicting the colour, is where it gets difficult!
