Color in transition metal complexes usually arises from $d-d$ transitions.

• To show color, the metal ion must have partially filled $d$-orbitals ($d^1$ to $d^9$).
• Zinc ($Z{n}^{2+}$): Electronic configuration is $3d^{10}$. Since the $d$-orbitals are completely filled, no electron can transition between $d$-energy levels ($e_g$ and $t_{2g}$). Therefore, Zinc salts like $ZnS{O}_4$ are colorless.
• $C{u}^{2+}$ ($3d^9$), $M{n}^{7+}$ ($d^0$ but shows color due to charge-transfer), and $C{r}^{6+}$ ($d^0$ but shows color due to charge-transfer) all form colored compounds.