Ferric alum gives deep red colour with ${\mathrm{NH}}_4\mathrm{SCN}$ due to the formation of :

Ferric alum gives a deep red colour with  ${\mathrm{NH}}_4\mathrm{SCN}$ due to the formation of $\mathrm{Fe}\left(\mathrm{SCN}{)}_3\right.$

The   $\mathrm{Fe}\left(\mathrm{SCN}{)}_3\right.$ formed when the  ${\mathrm{Fe}}^{3+}$ ion of the ferric atom reacts with the  ${\mathrm{NH}}_4\mathrm{SCN}$ and yields red colour Due to the formation of the thiocyanate iron complex  $\mathrm{Fe}\left(\mathrm{SCN}{)}_3\right.$ the reaction results in a solution with a blood-red colour.