cu²⁺ is coloured but cu⁺ is not and sc²⁺ is colured but sc³⁺ is not why????

We know that, color is shown by transition metal due to the transition between d-electrons which is known as d-d- transition for which the partly filled d orbitals must be involved in generating the color in some way.

Actually,the transition metals are defined as having partly filled d- orbitals, so, in case of scandium Sc with electronic configuration [Ar] 3d¹4s² if, 3 electrons are lost to form Sc⁺³ ions,there will be no d-electrons and hence no transition and no color change while the partially filled d-orbital with 1 electron in Sc⁺² ions will show a d-d transition and color.

Similarly in case of Cu [Ar] 3d¹⁰4s¹ , Cu⁺ ion will loose its 4s electron and filled 3d orbital will not have any transition and so no color shown but in Cu⁺² ion , the copper atom will lose two electrons and so it will have a 3d⁹ system which has an unpaired electron which would get transition and thus color will be shown.