Transition metal ions are colored due to d−d transition. Unpaired electrons in d orbitals are excited from lower energy level to higher energy level. For this, energy from visible region is absorbed.
Sc3+ has outer electronic configuration of 3d04s0 with zero unpaired electrons. Hence, it is colorless.
Ti3+ has outer electronic configuration of 3d14s0 with one unpaired electron. Hence, it is purple colored.