Q.Cu forms coloured salts while Zn forms colourless salts.

Step 1. Recall the d-block colour rule. Section 7.4.3 states that the ability to form coloured ions is a property of d-block elements with PARTIALLY-filled inner d-orbitals; elements like Zn, Cd and Hg, whose d subshell is completely filled ($ns^2(n-1)d^{10}$), do NOT show this property.

Step 2. Check Cu's typical ion. Copper commonly forms $Cu^{2+}$, with configuration $[Ar]3d^9$ — a partially-filled d subshell.

Step 3. Check Zn's typical ion. Zinc forms $Zn^{2+}$, with configuration $[Ar]3d^{10}$ — a completely-filled d subshell. …

Applying the d-block partially-filled-vs-completely-filled d-subshe …

• Attributing the colour difference to Cu and Zn being 'different metals' in general terms, without identifying the specific partially-filled vs completely-filled d-subshell mechanism. …