AgNO₃ reacts with Na₂S₂O₃ with a colour change :

On reaction of a hyposolution i.e. sodium thiosulphate with silver nitrate, silver thiosulphate is formed that goes through a colour change i.e. from white ppt to black ppt.

$$\begin{gathered} {\mathrm{Na}}_2{\mathrm{S}}_2{\mathrm{O}}_3+2{\mathrm{AgNO}}_3\to {\mathrm{Ag}}_2{\mathrm{S}}_2{\mathrm{O}}_3+2{\mathrm{NaNO}}_3 \\ {\mathrm{Ag}}_2{\mathrm{NO}}_3+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{Ag}}_2\mathrm{S}+{\mathrm{H}}_2{\mathrm{SO}}_4 \end{gathered}$$

$\text{ white }⟶\text{ yellow }⟶\text{ brown }⟶\text{ black }$