Ti3+(Z=22) Ions which have unpaired electrons exhibit colour in solution. Ti3+ has an outer electronic configuration of 4s∘3d′, i.e., 1 unpaired electron. Thus its solution will be coloured. Sc3*→d0 In case of La3+,4f0 configuration is present and in Lư, 4f14 is present. So, there is no possibility of f−f transition, hence these ions do not appear coloured.