Colour arises from d-d transitions — absorption of visible light promotes an electron from lower to higher d-orbital. Requirements: partially filled d-orbitals (at least one electron AND at least one vacancy). Therefore d^0 (Sc^3+, Ti^4+) and d^10 (Cu^+, Zn^2+) ions are colourless. Typical colours of aqueous M^2+ ions: Ti^3+(purple), V^3+(green), Cr^3+(green), Mn^2+(very pale pink), Fe^2+(green), Fe^3+(yellow-brown), Co^2+(pink), Ni^2+(green), Cu^2+(blue). Mn^2+ is nearly colourless because d^5 high spin has all transitions spin-forbidden.