$\mathrm{SnCl}{}_2$ solution is added dropwise to $\mathrm{HgCl}{}_2$ solution, and then in large excess. The sequence observed is

1. Ared $\mathrm{HgI}{}_2$, then yellow $\mathrm{HgO}$
2. Bwhite silky $\mathrm{Hg}{}_2\mathrm{Cl}{}_2$, which turns grey due to $\mathrm{Hg}$
3. Cgrey $\mathrm{Hg}$ first, then white $\mathrm{Hg}{}_2\mathrm{Cl}{}_2$
4. Dno change at any stage