$p$-Amino- $N, N$-dimethylaniline is added to a strongly acidic solution of $\mathbf{X}$. The resulting solution is treated with a few drops of aqueous solution of $\mathbf{Y}$ to yield blue coloration due to the formation of methylene blue. Treatment of the aqueous solution of $\mathbf{Y}$ with the reagent potassium hexacyanoferrate (II) leads to the formation of an intense blue precipitate. The precipitate dissolves on excess addition of the reagent. Similarly, treatment of the solution of $\mathbf{Y}$ with the solution of potassium hexacyanoferrate (III) leads to a brown coloration due to the formation of $\mathbf{Z}$.   The compound $\mathbf{Z}$ is