Acidified ${\text{K}}_2{\text{Cr}}_2{\text{O}}_7$ solution turns green when ${\text{Na}}_2{\text{SO}}_3$ is added to it. This is due to the formation of:

Potassium dichromate (${\text{Cr}}^{+6}$, orange) is a strong oxidizing agent. It oxidizes sulphite (${\text{SO}}_3^{2-}$) to sulphate (${\text{SO}}_4^{2-}$) and itself gets reduced to ${\text{Cr}}^{3+}$ ions, which are green. The product is chromium(III) sulphate.
$${\text{Cr}}_2{\text{O}}_7^{2-}+8{\text{H}}^{+}+3{\text{SO}}_3^{2-}\to 2{\text{Cr}}^{3+}+3{\text{SO}}_4^{2-}+4{\text{H}}_2\text{O}$$
The green color is due to ${\text{Cr}}_2({\text{SO}}_4{)}_3$.