Ti in [Ti(H2O)6]3+ is in +3 oxidation state. Sc in [Sc(H2O)6]3+ is in +3 oxidation state. The outer electronic configuration of Sc, Ti and their trivalent ions are,
SC: 3d1 4S2
SC3+ : 3d0
Ti: 3d2 4S2
Ti3+ : 3d1
Ti3+ has one unpaired electron in 3d orbital and they undergoes d-d transition. This electron can be promoted to a higher energy level by light absorption. Therefore [Ti(H2O)6]3+ is coloured. In the case of [Sc(H2O)6]3+ there is no electron in 3d orbital of Sc3+ , hence there is no possibility of light absorbance. Therefore [Sc(H2O)6]3+ is colourless.
