In the test of Nitrogen the Prussian blue color formed is of:

Formation of Nitrogen:-

1. Nitrogen in organic compounds is detected by Lassaigne's test
2. Here, a piece of $\mathrm{Na}$ metal is heated in a fusion tube with the organic compound.
3. Lassaigne's test is actually a test which is used for the elemental analysis for the determination of the elements such as Halogens, Nitrogen, and Sulphur in the organic compounds. In this Sodium metal $\left(\mathrm{Na}\right)$ is heated in the fusion tube where the organic compound would be present. Firstly all the elements are converted into the ionic state with the use of Sodium metal $\left(\mathrm{Na}\right)$.
4. Reactions involved in Lassaigne's test are:-

$\underset{\mathrm{Sodium}}{\mathrm{Na}}+\underset{\mathrm{Carbon}}{\mathrm{C}}+\underset{\mathrm{Nitrogen}}{\mathrm{N}}\to \underset{\mathrm{Sodium}\mathrm{cyanide}}{\mathrm{NaCN}}$

$\underset{\mathrm{Sodium}\mathrm{cyanide}}{6\mathrm{NaCN}}+\underset{\mathrm{Ferrous}\mathrm{sulphate}}{{\mathrm{FeSO}}_4}\to \underset{\mathrm{Sodium}\mathrm{ferrocyanide}}{{\mathrm{Na}}_4\left[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6\right]}+\underset{\mathrm{Sodium}\mathrm{sulphate}}{{\mathrm{Na}}_2{\mathrm{SO}}_4}$

$\underset{\mathrm{Sodium}\mathrm{ferrocyanide}}{{\mathrm{Na}}_4\left[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6\right]}+\underset{\mathrm{Iron}\mathrm{ion}}{{\mathrm{Fe}}^{+3}}\to \underset{\mathrm{Iron}\left(\mathrm{III}\right)\mathrm{hexacyanoferrate}}{{\mathrm{Fe}}_4[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6{]}_3}$

So, ${\mathrm{Fe}}_4[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6{]}_3$ formed is responsible for the Prussian blue colour obtained during the test of Nitrogen.