$\begin{aligned} & \mathrm{K}_2 \mathrm{Cr}_2 \mathrm{O}_7 \rightarrow \mathrm{Cr}^{+6}: 3 d^0 \\ & \left(\mathrm{NH}_4\right)_2\left[\mathrm{TiCl}_6\right] \rightarrow \mathrm{Ti}^{+4}: 3 d^0 \\ & \mathrm{VOSO}_4 \rightarrow \mathrm{V}^{+4}: 3 d^1 \\ & \mathrm{~K}_3\left[\mathrm{Cu}(\mathrm{CN})_4\right] \rightarrow \mathrm{Cu}^{+}: 3 d^{10}\end{aligned}$
$\mathrm{VOSO}_4$, due to the presence of one unpaired electron, is paramagnetic and coloured.