Addition of ${\mathrm{SnCl}}_2$ to ${\mathrm{HgCl}}_2$ gives a ppt. which is :

${\mathrm{SnCl}}_2$is added to ${\mathrm{HgCl}}_2$ to produce the white-to-gray transition precipitate.

$2{\mathrm{HgCl}}_2+{\mathrm{SnCl}}_2\to {\mathrm{Hg}}_2{\mathrm{Cl}}_2+{\mathrm{SnCl}}_4$

${\mathrm{Hg}}_2{\mathrm{Cl}}_2+{\mathrm{SnCl}}_2\to 2{\mathrm{Hg}}_{\mathrm{g}}+{\mathrm{SnCl}}_4$

The white ${\mathrm{Hg}}_2{\mathrm{Cl}}_2$that is formed subsequently becomes grey result in reduction.