Potassium dichromate

In alkaline medium orange dichromate changes to yellow chromate: $Cr_2O_7^{2-} + 2OH^- \rightarrow 2CrO_4^{2-} + H_2O$.
Acidified dichromate oxidises $H_2S$ to sulphur, which gives a milky appearance.
$Na_2Cr_2O_7$ is hygroscopic, so it cannot be weighed accurately. $K_2Cr_2O_7$ is not, and is used as a primary standard in volumetric analysis.
So the statement that $Na_2Cr_2O_7$ is preferred is not true.

Acidified potassium dichromate is a strong oxidising agent; $Na_2SO_3$ is a reducing agent.
Sulphite is oxidised to sulphate and orange $Cr_2O_7^{2-}$ (Cr in +6 state) is reduced to green $Cr^{3+}$.
Ionic equation: $Cr_2O_7^{2-} + 3SO_3^{2-} + 8H^+ \rightarrow 2Cr^{3+} + 3SO_4^{2-} + 4H_2O$
Molecular equation: $K_2Cr_2O_7 + 3Na_2SO_3 + 4H_2SO_4 \rightarrow 3Na_2SO_4 + K_2SO_4 + Cr_2(SO_4)_3 + 4H_2O$
The green colour is due to chromium(III) sulphate, $Cr_2(SO_4)_3$.