An unknown salt on treatment with dilute HCl evolves a colourless...

The evolution of a colourless gas with a rotten egg smell on treatment with dilute $\text{HCl}$ indicates the presence of the sulphide anion ($\text{S}^{2-}$). The gas evolved is hydrogen sulphide ($\text{H}_2\text{S}$).

As a confirmatory test, when an aqueous solution of a sulphide salt is treated with sodium nitroprusside ($\text{Na}_2[\text{Fe}(\text{CN})_5\text{NO}]$), a purple or violet colouration is obtained due to the formation of the complex $[\text{Fe}(\text{CN})_5\text{NOS}]^{4-}$.

$\text{S}^{2-} + [\text{Fe}(\text{CN})_5\text{NO}]^{2-} \rightarrow [\text{Fe}(\text{CN})_5\text{NOS}]^{4-}$ (Purple/Violet)

A black precipitate is formed in the lead acetate test for sulphide. A brown ring is the confirmatory test for nitrate. A yellow precipitate is formed in the confirmatory test for phosphate or arsenic.

Answer: Purple or violet colouration