Question

Why is $S{c}^{3+}$ colourless while ${Ti}^{3+}$ coloured ? (Atomic number $Sc=21,Ti=22$)

Answer 1

Electronic configuration of ${}_{21}{Sc}^{3+}$ is $\left[Ar\right]$ $3d^0$ and ${}_{22}{Ti}^{3+}$ is $\left[Ar\right]$ $3d^1$.
Therefore, due to absence of unpaired electron ${Sc}^{3+}$ is colourless and due to presence of one unpaired electron $d-d$ transition takes hence ${Ti}^{3+}$ is coloured.