Colour obtained in test of ${\mathrm{S}}_2{\mathrm{O}}_3^{2-}+\left[\mathrm{Ni}(\mathrm{en}{)}_3\right]{\left({\mathrm{NO}}_3\right)}_2$ is:

When thiosulphate reacts with nickel ethylenediamine nitrate leagents Crystelline ppt of which is $\left[\mathrm{Ni}{\left(\mathrm{en}\right)}_3\right]{\mathrm{S}}_2{\mathrm{O}}_3$ of violet colour.

$$\begin{gathered} {\left[\mathrm{Ni}(\mathrm{en}{)}_3\right]}^{2+}+{\mathrm{S}}_2{\mathrm{O}}_3^{2-}⟶\left[\mathrm{Ni}(\mathrm{en}{)}_3\right]{\mathrm{S}}_2{\mathrm{O}}_3\downarrow \\ (\mathrm{violet} \mathrm{ppt}.) \end{gathered}$$