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.