Ti3+(Z=22) Ions which have unpaired electrons exhibit colour in solution. Ti3+ has an outer electronic configuration of 4s03d1, i.e., 1 unpaired electron Thus its solution will be coloured Sc3+→d0. In case of La3+,4f0 configuration is present and in Lu3+,4f14 is present,So,there is no possibility of f - f transition, hence these ions do not appear coloured.