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.