When ammonia gas is passed through an alkaline solution of...

An alkaline solution of potassium tetraiodomercurate(II) ($\mathrm{K}_2[\mathrm{HgI}_4] + \mathrm{KOH}$) is known as Nessler's reagent.

When ammonia gas is passed through Nessler's reagent, it reacts to form a brown precipitate of the iodide of Millon's base.

The chemical reaction is given by:
$2\mathrm{K}_2[\mathrm{HgI}_4] + \mathrm{NH}_3 + 3\mathrm{KOH} \rightarrow \mathrm{HgO} \cdot \mathrm{Hg}(\mathrm{NH}_2)$\mathrm{I}$\downarrow + 7\mathrm{KI} + 2\mathrm{H}_2\mathrm{O}$

Thus, the precipitate is brown in colour and its chemical formula is $\mathrm{HgO} \cdot \mathrm{Hg}(\mathrm{NH}_2)\mathrm{I}$.

Answer: Brown, $\mathrm{HgO} \cdot \mathrm{Hg}(\mathrm{NH}_2)\mathrm{I}$